[libc++] test/libcxx/transitive_includes.sh.cpp dumps loads of unhelpful...

[libc++] test/libcxx/transitive_includes.sh.cpp dumps loads of unhelpful preprocessor output when it fails

Send stdout to dev/null since the preprocessor output isn't relevant to the test and is tons of noise when the test fails.

Reviewed By: Mordante, #libc, philnik

Differential Revision: https://reviews.llvm.org/D150056