[modules] Fix a #include cycle when building a module for our builtin headers.

xmmintrin.h includes emmintrin.h and vice versa if SSE2 is enabled. We break
this cycle for a modules build, and instead make the xmmintrin.h module
re-export the immintrin.h module. Also included is a fix for an assert in the
serialization code if a module exports another module that was declared later
in the same module map.

llvm-svn: 237321