[analyzer] Tighten up sanity checks on Objective-C property getter synthesis.

[analyzer] Tighten up sanity checks on Objective-C property getter synthesis.

If there are non-trivially-copyable types /other/ than C++ records, we
won't have a synthesized copy expression, but we can't just use a simple
load/return.

Also, add comments and shore up tests, making sure to test in both ARC
and non-ARC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199869 91177308-0d34-0410-b5e6-96231b3b80d8