[flang][MLIR] Hard crash when threading

When Flang (or rather MLIR) cannot spawn new threads, the compiler crashes with a stack trace. While it is probably okay to throw an error, the hard crash is not expected.

Here is a simple reproducer script. It is very artificial in the sense that it doesn't allow *any* threads to be spawned, but disabling threading for MLIR works around the problem.
```bash
#!/usr/bin/env bash

echo "flang: $(command -v flang)"
flang --version | head -1
echo

baseline=$(ps -u "$(id -u)" -L --no-headers | wc -l)
echo "current threads/processes for this user: $baseline"
echo

cat <repro.f90
end
EOF

echo "=== crash: repro.f90 is just the single line 'end', default flags, RLIMIT_NPROC=$baseline (zero headroom) ==="
bash -c "ulimit -u $baseline && flang -c repro.f90 -o /tmp/repro.o" 2>&1
echo

echo "=== workaround: -mmlir -mlir-disable-threading avoids the crash under the same limit ==="
bash -c "ulimit -u $baseline && flang -mmlir -mlir-disable-threading -c repro.f90 -o /tmp/repro.o" \
&& echo "compiled OK: /tmp/repro.o"
```

```
flang: /home/hmenke/LLVM-23.1.0-rc3-Linux-X64/bin/flang
flang version 23.1.0-rc3 (https://github.com/llvm/llvm-project 7196f931f212fc7c406066b2628a0ff4ea0ee344)

current threads/processes for this user: 499

=== crash: repro.f90 is just the single line 'end', default flags, RLIMIT_NPROC=499 (zero headroom) ===
LLVM ERROR: pthread_create failed: Resource temporarily unavailable
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0. Program arguments: /home/hmenke/LLVM-23.1.0-rc3-Linux-X64/bin/flang-23 -fc1 -triple x86_64-unknown-linux-gnu -emit-obj -fcolor-diagnostics -mrelocation-model pic -pic-level 2 -pic-is-pie -target-cpu x86-64 -resource-dir /home/hmenke/LLVM-23.1.0-rc3-Linux-X64/lib/clang/23 -fintrinsic-modules-path /home/hmenke/LLVM-23.1.0-rc3-Linux-X64/lib/clang/23/finclude/flang/x86_64-unknown-linux-gnu -mframe-pointer=all -o /tmp/repro.o -x f95 repro.f90
1. 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 flang-23 0x0000557bf3bd1bea llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 42
1 flang-23 0x0000557bf3bd257d
2 libc.so.6 0x0000147126c3ebf0
3 libc.so.6 0x0000147126c8c07c
4 libc.so.6 0x0000147126c3eb46 raise + 22
5 libc.so.6 0x0000147126c28833 abort + 211
6 flang-23 0x0000557bf3b861d1 llvm::report_fatal_error(llvm::Twine const&, bool) + 257
7 flang-23 0x0000557bf3bd352b
8 flang-23 0x0000557bf3bd34ae llvm::llvm_execute_on_thread_impl(void* (*)(void*), void*, std::optional) + 142
9 flang-23 0x0000557bf3d1d130
10 flang-23 0x0000557bf3d1b9a5 llvm::StdThreadPool::grow(int) + 261
11 flang-23 0x0000557bf3d1c994
12 flang-23 0x0000557bf3d1a5a1
13 flang-23 0x0000557bfa690ba0
14 flang-23 0x0000557bf3c26ef8
15 flang-23 0x0000557bfa48d520 mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int) + 1632
16 flang-23 0x0000557bfa491201 mlir::PassManager::run(mlir::Operation*) + 1409
17 flang-23 0x0000557bf3c1a5c2 Fortran::frontend::CodeGenAction::beginSourceFileAction() + 8306
18 flang-23 0x0000557bf3c12d03 Fortran::frontend::FrontendAction::beginSourceFile(Fortran::frontend::CompilerInstance&, Fortran::frontend::FrontendInputFile const&) + 355
19 flang-23 0x0000557bf3bfc532 Fortran::frontend::CompilerInstance::executeAction(Fortran::frontend::FrontendAction&) + 994
20 flang-23 0x0000557bf3c17d6e Fortran::frontend::executeCompilerInvocation(Fortran::frontend::CompilerInstance*) + 4126
21 flang-23 0x0000557bf29a4ca0 fc1_main(llvm::ArrayRef, char const*) + 2656
22 flang-23 0x0000557bf29a3658 main + 4392
23 libc.so.6 0x0000147126c295d0
24 libc.so.6 0x0000147126c29680 __libc_start_main + 128
25 flang-23 0x0000557bf28f4ed5 _start + 37
flang-23: error: unable to execute command: Aborted (core dumped)
flang-23: error: flang frontend command failed with exit code -2 (use -v to see invocation)
flang version 23.1.0-rc3 (https://github.com/llvm/llvm-project 7196f931f212fc7c406066b2628a0ff4ea0ee344)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/hmenke/LLVM-23.1.0-rc3-Linux-X64/bin
flang-23: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING CRASH REPRODUCER FILES TO THE BUG REPORT:
flang-23: note: diagnostic msg: /tmp/repro-99e0d3
flang-23: note: diagnostic msg: /tmp/repro-99e0d3.sh
flang-23: note: diagnostic msg:

********************

=== workaround: -mmlir -mlir-disable-threading avoids the crash under the same limit ===
compiled OK: /tmp/repro.o
```

Run the supplied Bash reproducer with the RLIMIT_NPROC setting, then compare it with -mmlir -mlir-disable-threading. Start at llvm::StdThreadPool::grow and llvm_execute_on_thread_impl in the stack trace, followed through MLIR's OpToOpPassAdaptor; done means flang no longer hard-crashes when thread creation fails and reports a handled failure.

Written by the indexing model from the issue text.