Introduce LocInfoType which is a Sema-specific implementation detail.

This is a Type subclass that can hold a DeclaratorInfo* when we have type source info coming
out of a declarator that we want to preserve. This is used only at the "border" of Parser/Sema for
passing/getting QualTypes, it does not participate in the type system semantics in any way.

llvm-svn: 79394