Apply --cxxopt to C++20 module interfaces

Copybara Import from bazelbuild/rules_cc#520

BEGIN_PUBLIC
Apply `--cxxopt` to C++20 module interfaces (bazelbuild#520)

C++20 module interfaces are not recognizable by fixed extensions, only by their appearance in the `module_interfaces` attribute.

Closes bazelbuild#520
END_PUBLIC

PiperOrigin-RevId: 829568265
Change-Id: Iee8da3eabdb27b4897e1d39d0ec56e904c3e1040

Author: fmeum

src/main/starlark/builtins_bzl/common/cc/compile/compile.bzl

@@ -558,6 +558,8 @@ def _create_scan_deps_action(
             copts = copts,
             cxxopts = cxxopts,
             label = source_label,
+            # Treat C++20 module interfaces as C++ source files regardless of their extension.
+            override_extension = extensions.CC_SOURCE[0],
         ),
     )
     compile_variables = _cc_internal.combine_cc_toolchain_variables(
@@ -1609,6 +1611,8 @@ def _create_compile_source_action(
         conlyopts = conlyopts,
         cxxopts = cxxopts,
         label = source_label,
+        # Treat C++20 module interfaces as C++ source files regardless of their extension.
+        override_extension = extensions.CC_SOURCE[0] if action_name == ACTION_NAMES.cpp20_module_compile else None,
     )
 
     compile_variables = get_specific_compile_build_variables(

src/main/starlark/builtins_bzl/common/cc/compile/compile_build_variables.bzl

@@ -385,8 +385,14 @@ def get_copts(
         conlyopts,
         copts,
         cxxopts,
-        label):
-    extension = "." + source_file.extension if source_file.extension else ""
+        label,
+        override_extension = None):
+    if override_extension != None:
+        extension = override_extension
+    elif source_file.extension:
+        extension = "." + source_file.extension
+    else:
+        extension = ""
     result = []
     result.extend(_copts_from_options(language, cpp_configuration, extension))
     result.extend(copts)