Remove redundant LLVM_HAS_RVALUE_REFERENCE_THIS and LLVM_LVALUE_FUNCTION defines

Now that VS2017 support has been dropped (D114639), the LLVM_HAS_RVALUE_REFERENCE_THIS define is always true and the LLVM_LVALUE_FUNCTION define is always enabled for ref-qualifiers.

This patch proposes we remove the defines and use the qualifiers directly.

Differential Revision: https://reviews.llvm.org/D118609