[clang] [clang][Driver] Add flag for setting SkipFunctionBodies (PR #100135)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> But I'll still ask the question: would it make sense to name this `-helpful-reproducers` or something more generic so we don't feel tempted to expose even more internal options this way?

This is tricky as some issues reproduce only when we skip function bodies and sometimes when we don't. Sometimes we need a PCM that skips function bodies and another file consuming it that does not (this is a normal operation mode of Clangd).
So even this particular flag does not bundle well with any other as we might need to set it independently of everything else.


+1 to Fangrui's suggestion to mention this is a `-cc1` flag in the subject line when committing this.
Otherwise, LGTM.



https://github.com/llvm/llvm-project/pull/100135