[flang] Add initial implementation for CPU_TIME

Add an implementation for CPU_TIME based on std::clock(), which should
be available on all the platforms that we support.

Also add a test that's basically just a sanity check to make sure we
return positive values and that the value returned at the start of some
amount of work is larger than the one returned after the end.

Differential Revision: https://reviews.llvm.org/D104019