[libc] Run libc tests unconditionally, even if there are no source changes.

Before this change, the test binaries were executed only if the deps
changed. This change makes the test binaries run unconditionally. This
is more in line with the rest of LLVM. Future patches will print test
statistics etc., much like the rest of LLVM.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D143627