[CMake] Module builds depend on target intrinsics_gen to be built first.

When compiling with modules, header A and B can be in the same module M.
B depends on intrinsics_gen and A doesn't. Compiling a source file #include-ing
header A, we implicitly request module M to be built. It puts header A and B in
the same TU and tries to build them. Since B depends on intrinsics_gen (which
might not be built yet) we run into build failures.

This should fix our modules buildbot.

Patch reviewed by Chris Bieneman.

llvm-svn: 274270