[clang-tidy] Fix false negative in readability-convert-member-functions-to-static

A nested class in a member function can erroneously confuse the check into
thinking that any CXXThisExpr found relate to the outer class, preventing any warnings.
Fix this by not traversing any nested classes.

Fixes https://github.com/llvm/llvm-project/issues/56765

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D130665