Support in hasDeclaration for types with getDecl()

Re-introducing r175532. The has_getDecl metafunction didn't compile with Visual
Studio. This revision approaches has_getDecl from a different angle that isn't
a problem for Visual Studio.

Added dedicated tests for the metafunction.

Reviewers: klimek


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176019 91177308-0d34-0410-b5e6-96231b3b80d8