[flang][OpenMP] Fortran block construct in OpenMP region

The following program leads to segmentation fault in the frontend:

```fortran
! omp_01.F90
module lw
implicit none
private
public :: stream
contains
subroutine stream(nx,ny,fsrc,fdst,w)
integer, intent(in), value :: nx, ny
real(8), intent(in) :: fsrc(0:ny+1,0:nx+1,0:8), w(-1:1,-1:1,1:8)
real(8), intent(inout) :: fdst(0:ny+1,0:nx+1,0:8)
integer :: i, j, k
!$omp parallel if(nx*ny > 100**2) default(private) &
!$omp shared(nx,ny,fsrc,fdst) firstprivate(w)
!$omp do collapse(2) schedule(static)
do k = 1, 8
do j = 1, nx
do i = 1, ny
block
real(8) :: delta
delta = sum(w(-1:1,-1:1,k)*fsrc(i-1:i+1,j-1:j+1,k))
fdst(i,j,k) = fsrc(i,j,k) + delta
end block
end do
end do
end do
!$omp end do
!$omp end parallel
end subroutine
end module
```

When compiled with `flang -c -fopenmp -fopenmp-version=51 omp_01.F90` I get,

```
Stack dump:
0. Program arguments: /opt/homebrew/Cellar/flang/21.1.8/libexec/flang -fc1 -triple arm64-apple-macosx26.0.0 -emit-obj -fcolor-diagnostics -mrelocation-model pic -pic-level 2 -target-cpu apple-m1 -target-feature +v8.4a -target-feature +aes -target-feature +altnzcv -target-feature +ccdp -target-feature +ccpp -target-feature +complxnum -target-feature +crc -target-feature +dotprod -target-feature +flagm -target-feature +fp-armv8 -target-feature +fp16fml -target-feature +fptoint -target-feature +fullfp16 -target-feature +jsconv -target-feature +lse -target-feature +neon -target-feature +pauth -target-feature +perfmon -target-feature +predres -target-feature +ras -target-feature +rcpc -target-feature +rdm -target-feature +sb -target-feature +sha2 -target-feature +sha3 -target-feature +specrestrict -target-feature +ssbs -fopenmp -fopenmp-version=51 -resource-dir /opt/homebrew/opt/llvm/lib/clang/21 -mframe-pointer=non-leaf -o /var/folders/5h/txm_8yhd2n721_w7gsrvlkj8p3bpj9/T/omp_01-062192.o -x f95 omp_01.F90
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0 libLLVM.dylib 0x0000000119eaf118 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1 libLLVM.dylib 0x0000000119eaf95c SignalHandler(int, __siginfo*, void*) + 372
2 libsystem_platform.dylib 0x000000018f8ad764 _sigtramp + 56
3 libFortranLower.dylib 0x0000000106cac6e8 void Fortran::lower::privatizeSymbol(Fortran::lower::AbstractConverter&, fir::FirOpBuilder&, Fortran::lower::SymMap&, llvm::SetVector, llvm::DenseSet>, 0u>&, llvm::SmallSet>&, Fortran::semantics::Symbol const*, mlir::omp::PrivateClauseOps*) + 124
4 libFortranLower.dylib 0x0000000106c2bcf4 Fortran::lower::omp::DataSharingProcessor::privatize(mlir::omp::PrivateClauseOps*) + 92
5 libFortranLower.dylib 0x0000000106c2a868 Fortran::lower::omp::DataSharingProcessor::processStep1(mlir::omp::PrivateClauseOps*) + 180
6 libFortranLower.dylib 0x0000000106c44df8 genOMPDispatch(Fortran::lower::AbstractConverter&, Fortran::lower::SymMap&, Fortran::semantics::SemanticsContext&, Fortran::lower::pft::Evaluation&, mlir::Location, llvm::SmallVector, 0u> const&, tomp::DirectiveWithClauses const*) + 14248
7 libFortranLower.dylib 0x0000000106c3fcb0 Fortran::lower::genOpenMPConstruct(Fortran::lower::AbstractConverter&, Fortran::lower::SymMap&, Fortran::semantics::SemanticsContext&, Fortran::lower::pft::Evaluation&, Fortran::parser::OpenMPConstruct const&) + 3612
8 libFortranLower.dylib 0x00000001067f72d0 void Fortran::common::log2visit::Log2VisitHelper<83ul, 93ul, void, Fortran::common::visitors::visit<(anonymous namespace)::FirConverter::genFIR(Fortran::lower::pft::Evaluation&, bool)::'lambda'(auto const&)>(auto&&) const::'lambda'(auto)>, std::__1::variant, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const> const&>(Fortran::common::visitors::visit<(anonymous namespace)::FirConverter::genFIR(Fortran::lower::pft::Evaluation&, bool)::'lambda'(auto const&)>(auto&&) const::'lambda'(auto)>&&, unsigned long, std::__1::variant, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, 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Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const, Fortran::parser::AllocateStmt const> const&) + 716
9 libFortranLower.dylib 0x00000001067ec560 (anonymous namespace)::FirConverter::lowerFunc(Fortran::lower::pft::FunctionLikeUnit&) + 492
10 libFortranLower.dylib 0x00000001067ebf8c Fortran::lower::LoweringBridge::lower(Fortran::parser::Program const&, Fortran::semantics::SemanticsContext const&) + 1100
11 libflangFrontend.dylib 0x0000000104f19f54 Fortran::frontend::CodeGenAction::beginSourceFileAction() + 1136
12 libflangFrontend.dylib 0x0000000104f172d0 Fortran::frontend::FrontendAction::beginSourceFile(Fortran::frontend::CompilerInstance&, Fortran::frontend::FrontendInputFile const&) + 356
13 libflangFrontend.dylib 0x0000000104e29798 Fortran::frontend::CompilerInstance::executeAction(Fortran::frontend::FrontendAction&) + 812
14 libflangFrontendTool.dylib 0x0000000104bfb408 Fortran::frontend::executeCompilerInvocation(Fortran::frontend::CompilerInstance*) + 4304
15 flang-21 0x0000000104bc75c0 fc1_main(llvm::ArrayRef, char const*) + 708
16 flang-21 0x0000000104bc6238 main + 700
17 dyld 0x000000018f4d9d54 start + 7184
flang-21: error: unable to execute command: Segmentation fault: 11
flang-21: error: flang frontend command failed due to signal (use -v to see invocation)
Homebrew flang version 21.1.8
Target: arm64-apple-darwin25.3.0
Thread model: posix
InstalledDir: /opt/homebrew/Cellar/flang/21.1.8/libexec
Configuration file: /opt/homebrew/Cellar/flang/21.1.8/libexec/flang.cfg
Configuration file: /opt/homebrew/etc/clang/arm64-apple-darwin25.cfg
flang-21: note: diagnostic msg:
```

The preprocessed source and reproducer,
* [omp_01-8ae189.txt](https://github.com/user-attachments/files/26113006/omp_01-8ae189.txt)
* [omp_01-8ae189.sh](https://github.com/user-attachments/files/26112999/omp_01-8ae189.sh)

It does not crash if I omit the `block`/`end block`.

Reproduce with `flang -c -fopenmp -fopenmp-version=51 omp_01.F90` using the supplied program. Start at `privatizeSymbol`, `DataSharingProcessor::privatize`, and `genOMPDispatch` in the stack trace, then trace the block construct through `genOpenMPConstruct`. Done means this reproducer no longer segfaults during compilation and has regression coverage for the failure.

Written by the indexing model from the issue text.