[flang][OpenMP] Selecting an assume variant from a metadirective hits llvm_unreachable in clause lowering

Flang aborts when a `metadirective` selects an `assume` variant carrying a `holds` clause. The direct spelling lowers fine.

```fortran
subroutine sel_assume(n)
integer :: n
!$omp metadirective when(implementation={vendor(llvm)}: assume holds(n>0)) otherwise(nothing)
print *, n
end subroutine
```

```
$ flang -fopenmp -fopenmp-version=52 -c repro.f90
Unimplemented: holds
UNREACHABLE executed at flang/lib/Lower/OpenMP/Clauses.cpp:1033!
```

Backtrace elided. The frame is reached from `Fortran::lower::omp::makeClause`.

Control, which emits exactly one `llvm.intr.assume`:

```fortran
subroutine plain_assume(n)
integer :: n
!$omp assume holds(n>0)
print *, n
!$omp end assume
end subroutine
```

`genOMP` for `OmpAssumeDirective` reads `holds` from the parse tree and creates `mlir::LLVM::AssumeOp`. Variant lowering instead routes every clause through `make()` in `flang/lib/Lower/OpenMP/Clauses.cpp`, where the `Holds` overload is `llvm_unreachable("Unimplemented: holds")`. Search for that function rather than the line number, which comes from my tree.

- Reproduces at `-fopenmp-version=52` with `otherwise(nothing)` and at `51` with `default(nothing)`, in both the standalone and `begin`/`end` spellings.
- The variant must win selection. A losing `assume` variant compiles.
- `-fsyntax-only` accepts the source, so only lowering fails.
- My builds have assertions enabled. Under `LLVM_UNREACHABLE_OPTIMIZE` a release build is undefined here instead and prints nothing. Untested.

Either the variant should lower like the direct spelling, or a diagnostic should reject it. An unreachable is not an appropriate failure mode for valid input.

Not a recent regression. The `llvm_unreachable` dates to 6d688601 in August 2024, and the abort reproduces on three builds carrying different unrelated local patches, none of which touches that file.

Start in flang/lib/Lower/OpenMP/Clauses.cpp by searching for the Holds overload in makeClause, then compare it with genOMP handling for OmpAssumeDirective. Compile the reproducer with flang -fopenmp -fopenmp-version=52 and check both metadirective spellings. Done means valid selected assume variants no longer reach llvm_unreachable and either lower consistently with direct assume or produce a diagnostic.

Written by the indexing model from the issue text.