Fix PR3766, a really nasty silent miscompilation case where we emitted

Fix PR3766, a really nasty silent miscompilation case where we emitted
a warning and then threw away the AST.  While I'm in there, tighten up the
code to actually reject completely bogus cases (sending a message to a
struct).  We still allow sending a message to an int, which doesn't make
sense but GCC allows it and is easy to support.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@66468 91177308-0d34-0410-b5e6-96231b3b80d8