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Fix "FIXME: does this lose qualifiers from the typedef??" in ASTContext::getTypedefType().

This change was fairly pervasive...nevertheless, here are the highlights:
- Change the type of Type::CanonicalType to TypeRef (was "Type *").
- Change the implementation of TypeRef::getCanonicalType() to work for typedefs.
- Make the implementation of Type::getCanonicalType private (only TypeRef should access). This
will force clients to use TypeRef::getCanonicalType (the correct version of the function). Since
TypeRef overloads "->", it was very easy to fall into this bug...
- Changed many references of "Type *" to "TypeRef"...when the raw type pointer is required, use t.getTypePtr().
- Changed all the *Type classes to take a TypeRef.
- Made the Type constructor protected (cleanup).
- Removed function Expr::getType().
- Convert functions in SemaExpr to use the above support. This fixed the "const" bug I was originally investigating.

I will follow this check-in up with a rename of TypeRef->QualType. I will also make sure the constructor does not default to 0 (which can lead to broken code...).

llvm-svn: 39394