Implement template instantiation for pointer, reference, and (some)

array types. Semantic checking for the construction of these types has
been factored out of GetTypeForDeclarator and into separate
subroutines (BuildPointerType, BuildReferenceType,
BuildArrayType). We'll be doing the same thing for all other types
(and declarations and expressions).

As part of this, moved the type-instantiation functions into a class
in an anonymous namespace. 

llvm-svn: 65663