[lldb/crashlog] Fix module loading for crashed thread behaviour

Before 27f27d15, the `crashlog` command would always load images even
if `-a` or `-c` was not set by the user.

Since that change, images are loaded only when one of these 2 flags are
set, otherwise, we fallback to parsing the symbols from the report and
load them into a `SymbolFileJSON`.

Although that makes it way faster than pulling binaries and debug
symbols from build records, that cause a degraded experience since none
of our users are used to set these 2 flags. For instance, that would
symbolicate the backtraces, however the users wouldn't see sources.

To address that change of behavior, this patch changes the default value
for the `-c|--crash-only` flag to `true`. On the other hand, thanks to
the move to `argparse`, we introduced a new `--no-only-crashed` flag
that will let the user force skipping loading any images, relying only
on the `SymbolFileJSON`.

This gives the users a good compromise since they would be able to see
sources for the crashed thread if they're available, otherwise, they'll
only get a symbolicated backtrace.

Differential Revision: https://reviews.llvm.org/D157850



Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>