[Sema] Don't crash on scanf on forward-declared enums.

This is valid in GNU C, which allows pointers to incomplete enums. GCC
just pretends that the underlying type is 'int' in those cases, follow
that behavior.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@279374 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 9c27c6bd260a3e4d531a2ebbcff339c4600da04f)
3 files changed