Updated checker for "return of address of stack variable" to look for

Updated checker for "return of address of stack variable" to look for
implicit casts from T to T& at the topmost part of the return-value expression.

This checking may be needed within EvalAddr later on.  We'll wait until
test cases show this kind of logic is necessary (as more C++ features are
implemented in clang).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@41493 91177308-0d34-0410-b5e6-96231b3b80d8