Re-enable the C++11 modules integration test on OS X

XCode 5.1.1 has shipped with fixed SDK headers.

This reverts commit r200256.

Fixes PR18322.
Move ObjC Cocoa.h header test to Headers

This tests for broad compatibility with platform SDK headers using the clang
driver and so belongs alongside the other header ingtegration tests.
Disallow driver use in more Sema tests

There are now only a handful of Sema tests remaining that use %clang in
SemaCXX, SemaObjC and SemaTemplate.