[clang] [InstallAPI] Add installapi specific options & diagnostics (PR #85100)

[Cyndy Ishida via cfe-commits]

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cyndyishida wrote:

Strictly speaking no, I don't. From looking at the git history, I think this is a pattern for supporting older includes through refactors. I figured to follow it for parity with the other DiagnosticKind files. 

https://github.com/llvm/llvm-project/pull/85100