Make WinCOFFObjectWriter.cpp's timestamp writing not use ENABLE_TIMESTAMPS

Make WinCOFFObjectWriter.cpp's timestamp writing not use ENABLE_TIMESTAMPS

LLVM_ENABLE_TIMESTAMPS controls if timestamps are embedded into llvm's
binaries. Turning it off is useful for deterministic builds.

r246905 made it so that the define suddenly also controls if the binaries that
the llvm binaries _create_ embed timestamps or not – but this shouldn't be a
configure-time option. r256203/r256204 added a driver option to toggle this on
and off, so this patch now passes this driver option in LLVM_ENABLE_TIMESTAMPS
builds so that if LLVM_ENABLE_TIMESTAMPS is set, the build of LLVM is
deterministic – but the built clang can still write timestamps into other
executables when requested.

This also allows removing some of the test machinery added in r292012 to work
around this problem.

See PR24740 for background.
http://reviews.llvm.org/D15783

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256958 91177308-0d34-0410-b5e6-96231b3b80d8