[HLSL] Run availability diagnostic on exported functions (#97352)
Implements availability diagnostic on `export` functions.
For shader libraries the HLSL availability diagnostic should run on all
entry points and export functions. Now that the `export` keyword is
implemented (llvm/llvm-project#96823), we can detect which functions are
exported and run the diagnostic on them.
Exported functions can be nested in namespaces and in export
declarations so we need to scan not just the current translation unit
but also namespace and export declarations contexts.
Fixes #92073