[clang] [cindex] Add API to query the class methods of a type (PR #123539)

[Trevor Laughlin via cfe-commits]

trelau wrote:

Not sure what happen, but there was a merge conflict with ReleaseNotes.rst where it looked like previous enhancements to liblang and Python bindings had been removed. To resolve the conflict I only left changes relevant to this PR.

https://github.com/llvm/llvm-project/pull/123539