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

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

What `clang_CXXMethod_getQualifiers` is supposed to return for a member function with an explicit object parameter (available since C++23)?
```cpp
struct S {
  void f() const&;
  void g(this const S& self);
};
```
I think the right approach here would be to ask for the first parameter (or object parameter, if that is available as an API), and interrogate its type whether qualifiers are present.

CC @AaronBallman 

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