[clang] [libclang] Add clang_CXXMethod_getQualifiers, clang_Cursor_isConstexpr, and clang_CXXMethod_isExplicitObjectMemberFunction (PR #183305)

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fscheidl wrote:

@Endilll Added `clang_CXXMethod_isExplicitObjectMemberFunction` and documented the behavior of `clang_CXXMethod_getQualifiers` for explicit object member functions

https://github.com/llvm/llvm-project/pull/183305