Basic support for representing elaborated type specifiers

directly in the AST.  The current thinking is to create these
only in C++ mode for efficiency.  But for now, they're not being
created at all; patch to follow.

This will let us do things like verify that tags match during
template instantation, as well as signal that an elaborated type
specifier was used for clients that actually care.

Optimally, the TypeLoc hierarchy should be adjusted to carry tag
location information as well.

llvm-svn: 81057