[Dexter] llvm-lit: always log DAP messages (#157145)

Buildbot cross-project-tests-sie-ubuntu sees sporadic test failures due to
missing "stackTrace" "source" "path". The "path" field is optional for
"source" according to DAP, so it's well formed. It works most of the time,
and doesn't consistently fail for any one test which is all strangely
inconsistent.

The failures couldn't be reproduced locally after running the feature_tests
in a loop for 3 hours. I haven't been able to work out why the "source" is
sometimes missing by just looking at LLDB code.

With this patch, we might be able to get clues about the flaky tests with
well timed log inspection.

Author: OCHyams

cross-project-tests/lit.cfg.py

@@ -121,7 +121,7 @@ def configure_dexter_substitutions():
         tools.append(
             ToolSubst(
                 "%dexter_lldb_args",
-                f'--lldb-executable "{lldb_dap_path}" --debugger lldb-dap',
+                f'--lldb-executable "{lldb_dap_path}" --debugger lldb-dap --dap-message-log=%t.dap.log',
             )
         )
 
@@ -148,7 +148,9 @@ def configure_dexter_substitutions():
         dexter_regression_test_c_builder = "clang"
         dexter_regression_test_cxx_builder = "clang++"
         dexter_regression_test_debugger = "lldb-dap"
-        dexter_regression_test_additional_flags = f'--lldb-executable "{lldb_dap_path}"'
+        dexter_regression_test_additional_flags = (
+            f'--lldb-executable "{lldb_dap_path}"  --dap-message-log=%t.dap.log'
+        )
         dexter_regression_test_c_flags = "-O0 -glldb -std=gnu11"
         dexter_regression_test_cxx_flags = "-O0 -glldb -std=gnu++11"