Remove LVALUE / RVALUE workarounds

Summary: LLVM_HAS_RVALUE_REFERENCE_THIS and LLVM_LVALUE_FUNCTION shouldn't be needed anymore because the minimum compiler versions support them.

Subscribers: jkorous, dexonsmith, cfe-commits, llvm-commits, hans, thakis, chandlerc, rnk

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D66240

llvm-svn: 368939