DiagnoseUnknownTypename always emits a diagnostic and returns true

Make it return void and delete the dead code in the parser that handled
the case where it might return false.  This has been dead since 2010
when John deleted Action.h.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211248 91177308-0d34-0410-b5e6-96231b3b80d8