[libc++][ci] Enable modules in the Runtimes build

The runtimes build has assertions enabled, which is necessary to catch
some of the modules-related issues we've been seeing recently. This
patch enables testing with modules in the runtimes build so as to cover
those cases.

In the future, a better solution would be to systematically use versions
of Clang that have assertions enabled. However, the Clangs we release
currently don't have assertions enabled by default, which causes a
challenge for the CI (we could try to build our own Clang from ToT with
assertions in the CI, but that poses some problems).

Differential Revision: https://reviews.llvm.org/D104252