[libc++] Avoid conflating stderr and stdout in the DSL

This is a workaround until https://reviews.llvm.org/D81892 is merged
and the internal Lit shell stops conflating error output with normal
output. Without this, any program that writes to stderr will trip up
the programOutput function, because it will pick up the '# command stderr:'
string and think it's part of the command's stdout.

rdar://81056048

Differential Revision: https://reviews.llvm.org/D107912