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

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>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121478 91177308-0d34-0410-b5e6-96231b3b80d8