Prevent lookup of subframework modules by name without parent framework

We were 'allowing' the following import
@import Sub;

where Sub is a subframework of Foo and we had a -F path inside
Foo.framework/Frameworks and no module map file for Sub. This would
later hit assertion failures in debug builds.

Now we should correctly diagnose this as a module not found error.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204368 91177308-0d34-0410-b5e6-96231b3b80d8