[Modules] Add testcase for builtins used in umbrella headers

This used to work before r284797 + r285152, which exposed something
interesting; some users include builtins from umbrella headers.

Clang should emit a warning to warn users this is not a good practice
and umbrella headers shouldn't get the
implicitly-add-the-builtin-version behavior for builtin header names.

While we're not there, add the testcase to represent the way it
currently works.

llvm-svn: 285377