[PATCH] Modules - fix exclude header with umbrella directory

[Ben Langmuir]

  I chatted with Doug and we agreed this looks good to remove.

  You'll need to fix this test, probably by just removing the exclude header "y/b.h", which looks wrong:
  FAIL: Clang :: Modules/exclude-header.c (3575 of 7161)

http://reviews.llvm.org/D3090