[modules] Make the tweak to avoid circular inclusion of emmintrin.h and

xmmintrin.h a bit more directed. If for whatever reason modules are enabled but
we textually include one of these headers, don't deploy the special case for
modules. To make this work cleanly, extend __building_module to be defined
even when modules is disabled.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@266945 91177308-0d34-0410-b5e6-96231b3b80d8