Improve our handling of cv-qualifiers in Objective-C pointer

conversions. Previously, we would end up collapsing qualification
conversions into the Objective-C pointer conversion step, including
(possibly) stripping qualifiers that shouldn't be removed.

This generalizes BuildSimilarlyQualifiedPointerType() to also work on
Objective-C object pointers, then eliminates the (redundant, not
totally correct) BuildSimilarlyQualifiedObjCObjectPointerType()
function.

Fixes <rdar://problem/8714395>.

llvm-svn: 120607