Removing has_getDecl (added in r175532). It cause a build break for MSVC, and was not yet being used in the codebase. If we start using std::enable_if, we can look into resurrecting the idea in a more portable fashion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175837 91177308-0d34-0410-b5e6-96231b3b80d8