Make module self-import an error

Ideally, importing Foo.a from Foo.b would "do the right thing", but
until it does, this patch makes it an error rather than allow it to
silently be ignored.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207948 91177308-0d34-0410-b5e6-96231b3b80d8