[analyzer] Don't assert on a temporary of pointer-to-member type.

While we don't do anything intelligent with pointers-to-members today,
it's perfectly legal to need a temporary of pointer-to-member type to, say,
pass by const reference. Tweak an assertion to allow this.

PR15742 and PR15747

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179563 91177308-0d34-0410-b5e6-96231b3b80d8