[libc++] Make the %run substitution closer to how .pass.cpp tests are executed

Before this patch, the %run substitution did not contain the same
environment variables as normal `pass.cpp` tests. It also didn't
have the right working directory and the script wasn't aware of
potential file dependencies.

With this change, the combination of %build and %run in a .sh.cpp script
should match how pass.cpp tests are actually executed much more closely.