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

[mib]

Summary

Before 27f27d15f, 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.

Signed-off-by: Med Ismail Bennani <[e-mail]>

[JDevlieghere]

LGTM

lldb/examples/python/crashlog.py

arg_parser.add_argument(
         dest="debug_delay",
         metavar="NSEC",
         help="pause for NSEC seconds for debugger",
         default=0,
     )
     arg_parser.add_argument(
         "--crashed-only",
         "-c",
-        action="store_true",
+        action=argparse.BooleanOptionalAction,
         dest="crashed_only",
         help="only symbolicate the crashed thread",
-        default=False,
+        default=True,
     )
     arg_parser.add_argument(
         "--disasm-depth",
         "-d",
         type=int,
         dest="disassemble_depth",
         help="set the depth in stack frames that should be disassembled",
         default=1,