Revise the SplitQualType interface to make it its own thing instead of

a typedef of std::pair.  This slightly improves type-safety, but mostly
makes code using it clearer to read as well as making it possible to add
methods to the type.

Add such a method for efficiently single-step desugaring a split type.
Add a method to single-step desugaring a locally-unqualified type.
Implement both the SplitQualType and QualType methods in terms of that.

Also, fix a typo ("ObjCGLifetime").

llvm-svn: 150028