[clang] [Wunsafe-buffer-usage] Fix false positive when const sized array is indexed by const evaluatable expressions (PR #119340)

[Malavika Samak via cfe-commits]

malavikasamak wrote:

@nico Thanks for the reproducer. I have relanded this change with the fix for the failing assert. https://github.com/llvm/llvm-project/commit/2a8c12b29f8dc777a62868512bed1a2dae1ef8b2 

https://github.com/llvm/llvm-project/pull/119340