Changes how the TypeLoc traverser invokes the Type traverser: before,

VisitFooTypeLoc() calls VisitFooType(); now, TraverseFooTypeLoc()
calls WalkUpFromFooType(). This allows clients that override
WalkUpFromFooType() to continue to work.  It also preserves the
property that Visit*() in the base visitor class is a no-op (s.t. a
subclass doesn't have to call Base::Visit*() when overriding
Visit*()).

Also fixes some typos in comments.
         
Also added a missing getDerived() inside
TraverseQualifiedTypeLoc(). The call is needed in case a subclass
overrides TraverseTypeLoc().

Reviewed by nlewycky and csilvers.

llvm-svn: 107816