[cross-project-tests][formatters] Add a LIT feature that tests for a compatible LLDB version (#174812)

Most of this logic is similar to how we add the
`gdb-clang-incompatibility` attribute.

Some LLVM LLDB formatters will rely on LLDB features not available in
older versions of LLDB. This feature will allow those tests to add a
`REQUIRES: lldb-formatters-compatibility`, which will then mark the test
`UNSUPPORTED` for incompatible LLDB versions. I picked `19.0` pretty
arbitrarily based on when approximately the
`SBType::FindDirectNestedType` API was added to LLDB (which a future
formatter will require).

cross-project-tests/lit.cfg.py

@@ -109,6 +109,7 @@ if lldb_dap_path is not None:
 if llvm_config.use_llvm_tool("llvm-ar"):
     config.available_features.add("llvm-ar")
 
+
 def configure_dexter_substitutions():
     """Configure substitutions for host platform and return list of dependencies"""
     # Produce dexter path, lldb path, and combine into the %dexter substitution
@@ -289,6 +290,44 @@ def get_clang_default_dwarf_version_string(triple):
     return match.group(1)
 
 
+def get_lldb_version_string():
+    """Return LLDB's version string, or None if lldb cannot be found or the
+    --version output is formatted unexpectedly.
+    """
+    try:
+        lldb_vers_lines = (
+            subprocess.check_output(["lldb", "--version"]).decode().splitlines()
+        )
+    except:
+        return None
+    if len(lldb_vers_lines) < 1:
+        print("Unkown LLDB version format (too few lines)", file=sys.stderr)
+        return None
+    match = re.search(r"lldb.*[ -]((\d|\.)+)", lldb_vers_lines[0].strip())
+    if match is None:
+        print(f"Unkown LLDB version format: {lldb_vers_lines[0]}", file=sys.stderr)
+        return None
+    return match.group(1)
+
+
+def set_lldb_formatters_compatibility_feature():
+    lldb_version_string = get_lldb_version_string()
+    if lldb_version_string is None:
+        return False
+
+    try:
+        from packaging import version
+    except:
+        lit_config.fatal("Running lldb tests requires the packaging package")
+        return False
+
+    if version.parse(lldb_version_string) < version.parse("1900.0"):
+        return False
+
+    config.available_features.add("lldb-formatters-compatibility")
+    return True
+
+
 # Some cross-project-tests use gdb, but not all versions of gdb are compatible
 # with clang's dwarf. Add feature `gdb-clang-incompatibility` to signal that
 # there's an incompatibility between clang's default dwarf version for this
@@ -310,6 +349,12 @@ if dwarf_version_string and gdb_version_string:
                 file=sys.stderr,
             )
 
+if not set_lldb_formatters_compatibility_feature():
+    print(
+        f"Marking some LLDB LLVM data-formatter tests as unsupported: using version {get_lldb_version_string()} whereas a version >= 1900.0 is required",
+        file=sys.stderr,
+    )
+
 llvm_config.feature_config([("--build-mode", {"Debug|RelWithDebInfo": "debug-info"})])
 
 # Allow 'REQUIRES: XXX-registered-target' in tests.