[clang] Return available function types for BindingDecls. (PR #102196)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Do you have a C++ source file example which presents misbehaviour when compiled through clang, which this change fixes?

If so, you can reduce that example and add is as a regression test. These go in `clang/test/` and are basically composed of C++ source and some annotations to verify we are doing the right thing with them.

If you can post the example, we can give further advice.

https://github.com/llvm/llvm-project/pull/102196