Revert rL349624 : Let TableGen write output only if it changed,
instead of doing so in cmake, attempt 2
Differential Revision: https://reviews.llvm.org/D55842
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As discussed on PR43385 this is causing Visual Studio msbuilds to
perpetually rebuild all tablegen generated files
Pull in r373664 from upstream llvm trunk (by Nico Weber):
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.
This should fix the scenario where an incremental build from before
r353358 (the clang 9.0.0 upgrade) to r353358 or later fails to update
the timestamp of the generated lib/clang/headers/arm_fp16.h header.
After such a build, installing world from read-only source and object
directories would attempt to generate the header again, leading to
"clang-tblgen: error opening arm_fp16.h.d:Read-only file system".
Reported by: avg, np
PR: 241402
MFC after: 1 month
X-MFC-With: r353358
f1eeb689ab