[libc++] Pickle substitutions to pass them to dsl.sh.py

This is less brittle than hand-picking the substitutions that we
pass to the test, since a config could theorically use non-base
substitutions as well (such as defining %{flags} in terms of another
substitution like %{include}).

Also, print the decoded substitutions, which makes it much easier
to debug the test when it fails.

Differential Revision: https://reviews.llvm.org/D111179