Issue a warning instead of fatal errors when checks for libatomic fail.

This should fix PR26631, PR26622 and has the nice property that the addition
of the CheckLibcxxAtomic.cmake module acts as an NFC on the platforms of the
reporters (at least for the time being).

As these bug reports explain, CMake fails the atomic check because the
include headers might not exist in the host environment. We could
potentially point to the headers provided by libcxx itself.

llvm-svn: 260961