[lit] Report line number for failed RUN command

[lit] Report line number for failed RUN command

When debugging test failures with -vv (or -v in the case of the
internal shell), this makes it easier to locate the RUN line that
failed.  For example, clang's test/Driver/linux-ld.c has 892 total RUN
lines, and clang's test/Driver/arm-cortex-cpus.c has 424 RUN lines
after concatenation for line continuations.

When reading the generated shell script, this also makes it easier to
locate the RUN line that produced each command.

To support reporting RUN line numbers in the case of the internal
shell, this patch extends the internal shell to support the null
command, ":", except pipelines are not supported.

Reviewed By: asmith, delcypher

Differential Revision: https://reviews.llvm.org/D44598

llvm-svn: 330755