[lib/Headers] Break the module import cycle between _Builtin_intrinsics.sse and _Builtin_intrinsics.sse2

Module "sse" implicitly exports module "sse2".
This is bad because we also have module "sse2" export module "sse" (as intended) so we end up with a cycle
in the module import graph:
1. sse2 -> (also imports) sse
2. sse -> (also imports) sse2

To eliminate the cycle remove 2.; importing module "sse2" will also import module "sse", but just importing
module "sse" will not also import module "sse2".

rdar://13240552