Fix Objective-C implicit property synthesis for C++ classes so we use valid

Fix Objective-C implicit property synthesis for C++ classes so we use valid
source locations in places where it is necessary for diagnostics.  By itself,
this causes assertions, so while I'm here, also fix property synthesis
for properties of C++ class type so we use so we properly set up a scope
and mark variable declarations.

<rdar://problem/12514189>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166219 91177308-0d34-0410-b5e6-96231b3b80d8