Let TableGen write output only if it changed, instead of doing so in cmake, attempt 2

Let TableGen write output only if it changed, instead of doing so in cmake, attempt 2

This relands r330742:
"""
Let TableGen write output only if it changed, instead of doing so in cmake.

Removes one subprocess and one temp file from the build for each tablegen
invocation.

No intended behavior change.
"""

In particular, if you see rebuilds after this change that you didn't see
before this change, that's unintended and it's fine to revert this change
again (but let me know).

r330742 got reverted because some people reported that llvm-tblgen ran on every
build after it.  This could happen if the depfile output got deleted without
deleting the main .inc output. To fix, make TableGen always write the depfile,
but keep writing the main .inc output only if it has changed. This matches what
we did in cmake before.

Differential Revision: https://reviews.llvm.org/D55842

llvm-svn: 349624