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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Apologies for the long delay, but @Endilll and I were finally able to meet to discuss this PR in more detail during a 1:1 today.

> I'd expect clang_CXXMethod_getQualifiers to return the actual method qualifiers (the ones after the parameter list) - the naming itself implies we're querying qualifiers of the method, not of its parameters.

I tend to agree, but it's not a strongly held position. We often make a distinction between semantic and syntactic forms by having `isFoo()` for the semantic form and `isFooAsWritten()` for the syntactic form. So that is one possibility here, but we don't really use that naming convention anywhere else in Index.h and all the existing names seem to largely be `AsWritten` in terms of behavior. I think I'd be in favor of naming it `clang_CXXMethod_getQualifiersAsWritten()` to leave the door open to a semantic-based interface in the future if we need to do that. WDYT?

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