Remove LLVM_ENABLE_TIMESTAMPS

Summary:
As per the discussion on LLVM-dev this patch proposes removing LLVM_ENABLE_TIMESTAMPS.

The only complicated bit of this patch is the Windows support. On windows we used to log an error if /INCREMENTAL was passed to the linker when timestamps were disabled.

With this change since timestamps in code are always disabled we will always compile on windows with /Brepro unless /INCREMENTAL is specified, and we will log a warning when /INCREMENTAL is specified to notify the user that the build will be non-deterministic.

See: http://lists.llvm.org/pipermail/llvm-dev/2016-May/098990.html

Reviewers: bogner, silvas, rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19892

llvm-svn: 268670