It's kindof silly that ExtQuals has an ASTContext&, and we can use that

space better.  Remove this reference.  To make that work, change some APIs
(most importantly, getDesugaredType()) to take an ASTContext& if they
need to return a QualType.  Simultaneously, diminish the need to return a
QualType by introducing some useful APIs on SplitQualType, which is
just a std::pair<const Type *, Qualifiers>.

llvm-svn: 121478