[IR] Split Intrinsics.inc into enums and implementations

Implements PR34259

Intrinsics.h is a very popular header. Most LLVM TUs care about things
like dbg_value, but they don't care how they are implemented. After I
split these out, IntrinsicImpl.inc is 1.7 MB, so this saves each LLVM TU
from scanning 1.7 MB of source that gets pre-processed away.

It also means we can modify intrinsic properties without triggering a
full rebuild, but that's probably less of a win.

I think the next best thing to do would be to split out the target
intrinsics into their own header. Very, very few TUs care about
target-specific intrinsics. It's very hard to split up the target
independent intrinsics like llvm.expect, assume, and dbg.value, though.

llvm-svn: 335407