[clang] [Clang][HTO] Add clang attribute for propagating llvm-level information (PR #83059)

[William Moses via cfe-commits]

wsmoses wrote:

As another option, what if this always emits string attributes prefixed with "clang_". And any other tools that semantic assurances for what they do with it. Thus the attribute won't be tied to any LLVM attributes.

https://github.com/llvm/llvm-project/pull/83059