If the headermap maps the filename to a framework include ("Foo.h" -> "Foo/Foo.h"),
continue header lookup using the framework include as filename.

This allows us to conveniently treat
  #import "Foo.h"
as an implicit module import if we can resolve "Foo/Foo.h" as such.

rdar://16042979

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201419 91177308-0d34-0410-b5e6-96231b3b80d8