Remove getVTT* in favor of getStructorImplicitParameter*

No functionality change.  The only remaining uses were in ItaniumCXXABI,
which knows the implicit parameter is a VTT.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197189 91177308-0d34-0410-b5e6-96231b3b80d8