[clang] [libclang/python] Add `Cursor.from_translation_unit` (PR #140496)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

Hmm, so the reason we have `Cursor.from_source_location` is that SourceLocation doesn't hold a reference to TU. Yeah, I guess we don't need this.

https://github.com/llvm/llvm-project/pull/140496