[CFG] [analyzer] Work around a disappearing CXXBindTemporaryExpr.

Sometimes template instantiation causes CXXBindTemporaryExpr to be missing in
its usual spot. In CFG, temporary destructors work by relying on
CXXBindTemporaryExprs, so they won't work in this case.

Avoid the crash and notify the clients that we've encountered an unsupported AST
by failing to provide the ill-formed construction context for the temporary.

Differential Revision: https://reviews.llvm.org/D44955

llvm-svn: 328895