[Modules] Rework r274270. Let Clang targets depend on intrinsics_gen.

This gets rid of almost LLVM targets unconditionally depending on intrinsic_gen.

Clang's modules still have weird dependencies and hard to remove intrinsics_gen in better way.
Then, it'd be better to give whole clang targets depend on intrinsic_gen.

llvm-svn: 308844