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.