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

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>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@120607 91177308-0d34-0410-b5e6-96231b3b80d8