[flang] [clang] [mlir] [clang-tools-extra] [libcxx] [libc] [compiler-rt] [llvm] [AsmWriter] Ensure getMnemonic doesn't return invalid pointers (PR #75783)

[Tomas Matheson via cfe-commits]

https://github.com/tmatheson-arm approved this pull request.

LGMT, surprised this didn't blow up earlier.

https://github.com/llvm/llvm-project/pull/75783