CMake: fix compiler feature detection

add_flag_if_supported() and add_flag_or_print_warning() were effectively
no-ops, just returning the value of the first result (usually
'-fno-omit-frame-pointer') for all subsequent checks for different flags.

Due to the way CMake caches feature detection results, we need to provide
symbolic variable names which will persist the cached results. This commit
fixes feature detection using these two macros.

The feature checks now run and get stored correctly, and the correct output can
be observed in configure logs:

  -- Performing Test C_SUPPORTS_FPIC
  -- Performing Test C_SUPPORTS_FPIC - Success
  -- Performing Test CXX_SUPPORTS_FPIC
  -- Performing Test CXX_SUPPORTS_FPIC - Success

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212586 91177308-0d34-0410-b5e6-96231b3b80d8
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