Fix noexcept for delete expressions.

Using "delete" on a pointer to an incomplete type can't throw.
While I'm here, clean up the signature of the canCalleeThrow() helper.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184810 91177308-0d34-0410-b5e6-96231b3b80d8