[clang] [llvm] [clang][CodeGen] Construct function type for callgraph from function definition (PR #212863)

[Roland McGrath via cfe-commits]

https://github.com/frobtech approved this pull request.

Test coverage and results lgtm.  There is probably some more maintainable long-term way to share the code / logic about argument promotion with the apparent source of truth in `Sema::DefaultArgumentPromotion`, and comments in the  new code should point to that.  IMHO unifying with that code or otherwise formalizing the correspondence between the expression-based and type-based implementations is refinement that can happen incrementally but should not be left undone for the long term.

https://github.com/llvm/llvm-project/pull/212863