Warning for framework include violation from Headers to PrivateHeaders

Framework vendors usually layout their framework headers in the
following way:

Foo.framework/Headers -> "public" headers
Foo.framework/PrivateHeader -> "private" headers

Since both headers in both directories can be found with #import
<Foo/some-header.h>, it's easy to make mistakes and include headers in
Foo.framework/PrivateHeader from headers in Foo.framework/Headers, which
usually configures a layering violation on Darwin ecosystems. One of the
problem this causes is dep cycles when modules are used, since it's very
common for "private" modules to include from the "public" ones; adding
an edge the other way around will trigger cycles.

Add a warning to catch those cases such that:

./A.framework/Headers/A.h:1:10: warning: public framework header includes private framework header 'A/APriv.h'
#include <A/APriv.h>
         ^

rdar://problem/38712182

llvm-svn: 335542