CMake: make __DATE__, __TIME__ etc. macro usage an error

When LLVM_ENABLE_TIMESTAMPS has been disabled we can prevent the preprocessor
from embedding dates, times and file timestamps.

There are a few motivations for this:

  1) Validate the recent CMake feature detection bugfix from LLVM r212586 with
     a flag that's not actually available everywhere.

  2) Dogfood clang's new -Wdate-time warning from r210511 when bootstrapping.

  3) Encourage reproducible builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212587 91177308-0d34-0410-b5e6-96231b3b80d8