Fix ninja check-lldb

Ninja apparently has issues with commands writing nul characters '\0' to stdout. When it
encounters a nul character, the rest of the output is skipped (I will file a bug with ninja about
that). This breaks the linux buildbot since it parses the ninja check-lldb output to get a list
of failures.

Recently, we have started outputting nul characters in lldb-server tests. This is probably
a bug in itself, but I am not addressing that now. This is just a workaround commit, which
prevents the nul characters from appearing in the output, and gets the buildbot operational again.

llvm-svn: 232805