[lit] add a -vv option to echo all executed commands.

Debugging LIT scripts can be rather painful, as LIT directly does not
specify which line has failed.
Rather, FileCheck is expected to report the failing location, but it can
be often ambiguous if multiple commands are tested against the same
prefix.  This change adds a -vv option, which echoes all output.
Then detecting the error becomes straightforward: last printed line is
the failing one.

Of course, it could be desired to try to get failing line number
directly from bash, but it involves excessive hacks on older bash
versions (cf.
https://stackoverflow.com/questions/24398691/how-to-get-the-real-line-number-of-a-failing-bash-command)

Differential Revision: https://reviews.llvm.org/D35330

llvm-svn: 307938