[MC, COFF] Unbreak support for COFF timestamps

[MC, COFF] Unbreak support for COFF timestamps

Support for COFF timestamps was unintentionally broken in r246905 when
it was conditionally available depending on whether or not LLVM was
configured with LLVM_ENABLE_TIMESTAMPS.  However, Config/config.h was
never included which essentially broke the feature.  Due to lax testing,
the breakage was never identified until we observed strange failures
during incremental links of Chromium.

This issue is resolved by simply including Config/config.h in
WinCOFFObjectWriter and teaching lit that the MC/COFF/timestamp.s test
is conditionally supported depending on LLVM_ENABLE_TIMESTAMPS.  With
this in place, we can strengthen the test to ensure that it will not
accidentally get broken in the future.

This fixes PR25891.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256137 91177308-0d34-0410-b5e6-96231b3b80d8