[clang] [CIR] Function type return type improvements (PR #128787)

[David Olsen via cfe-commits]

dkolsen-pgi wrote:

This combines https://github.com/llvm/clangir/pull/1249 (fix handling of void return) and https://github.com/llvm/clangir/pull/1391(trailing return type) into a single upstream PR.


https://github.com/llvm/llvm-project/pull/128787