[libc++][cmake] Improves benchmark build.

The CMake dependencies don't properly list the libc++ headers. When a
libc++ header is modified the affected benchmarks aren't rebuild. This
makes testing benchmarks tricky and may cause accidentally not using the
latest modifications during testing. This change causes CMake to
determine the proper dependencies.

This shouldn't affect the CI build.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D113419