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