[PATCH] Modules - fix exclude header with umbrella directory

[Richard Smith]

  I'm hesitant to say this LGTM, since this code was clearly added deliberately, but I'm having a hard time understanding why we'd want strange (and subtly-broken) rules for umbrella directories. Why is the model not simply that they implicitly generate a bunch of submodules?

  Also, this patch should have an accompanying testcase showing that it fixes the behavior of `exclude header` when combined with umbrella headers.

http://reviews.llvm.org/D3090