[libc++] Use add_lit_testsuite to register the libc++ test suite

The Runtimes build uses variables set by add_lit_testsuite to discover
tests suites to run.

Differential Revision: https://reviews.llvm.org/D97913

GitOrigin-RevId: 6427c53940a3d66ba90f72075554431066c5919d
1 file changed