[lit] Drop "Script:", make -v and -a imply -vv

This patch and D156954 were discussed in
<https://discourse.llvm.org/t/rfc-improving-lits-debug-output/72839>.

**Motivation**: -a shows output from all tests, and -v shows output
from just failed tests.  Without this patch, that output from each
test includes a section called "Script:", which includes all shell
commands that lit has computed from RUN directives and will attempt to
run for that test.  The effect of -vv (which also implies -v if
neither -a or -v is specified) is to extend that output with shell
commands as they are executing so you can easily see which one failed.

For example, when using lit's internal shell and -vv:

```
Script:
--
: 'RUN: at line 1'; echo hello world
: 'RUN: at line 2'; 3c40 hello world
: 'RUN: at line 3'; echo hello world
--
Exit Code: 127

Command Output (stdout):
--
$ ":" "RUN: at line 1"
$ "echo" "hello" "world"
hello world

$ ":" "RUN: at line 2"
$ "3c40" "hello" "world"
'3c40': command not found
error: command failed with exit status: 127

--
```

Notice that all shell commands that actually execute appear in the
output twice, once for "Script:" and once for -vv.  Especially for
tests with many RUN directives, the result is noisy.  When searching
through the output for a particular shell command, it is easy to get
lost and mistake shell commands under "Script:" for shell commands
that actually executed.

**Change**: With this patch, a test's output changes in two ways.
First, the "Script:" section is never shown.  Second, omitting -vv no
longer disables printing of shell commands as they execute.  That is,
-a and -v imply -vv, and so -vv is deprecated as it is just an alias
for -v.

**Secondary motivation**: We are also working to introduce a PYTHON
directive, which can appear between RUN directives.  How should PYTHON
directives be represented in the "Script:" section, which has
previously been just a shell script?  We could probably think of
something, but adding info about PYTHON directive execution in the -vv
trace seems more straight-forward and more useful.

(This patch also removes a confusing point in the -vv documentation:
at least when using bash as an external shell, -vv echoes commands to
the shell's stderr not stdout.)

Reviewed By: awarzynski, Endill, ldionne, MaskRay

Differential Revision: https://reviews.llvm.org/D154984