Reland r349624: Let TableGen write output only if it changed, instead of doing so...

Reland r349624: Let TableGen write output only if it changed, instead of doing so in cmake

Move the write-if-changed logic behind a flag and don't pass it
with the MSVC generator. msbuild doesn't have a restat optimization,
so not doing write-if-change there doesn't have a cost, and it
should fix whatever causes PR43385.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373664 91177308-0d34-0410-b5e6-96231b3b80d8