Get rid of the non-strict use of -fmodules-decluse.

For layering checks with explicit modules, we only want
`-fmodules-strict-decluse`; it's not useful to let headers
without a known module map sneak through.

PiperOrigin-RevId: 375825780

swift/internal/compiling.bzl

@@ -365,20 +365,6 @@ def compile_action_configs(
 
     #### Flags controlling how Swift/Clang modular inputs are processed
 
-    def c_layering_check_configurator(prerequisites, args, *, strict):
-        # We do not enforce layering checks for the Objective-C header generated
-        # by Swift, because we don't have predictable control over the imports
-        # that it generates. Due to modular re-exports (which are especially
-        # common among system frameworks), it may generate an import declaration
-        # for a particular symbol from a different module than the Swift code
-        # imported it from.
-        if not prerequisites.is_swift_generated_header:
-            args.add(
-                "-Xcc",
-                "-fmodules-strict-decluse" if strict else "-fmodules-decluse",
-            )
-        return None
-
     action_configs += [
         # Treat paths in .modulemap files as workspace-relative, not modulemap-
         # relative.
@@ -459,27 +445,7 @@ def compile_action_configs(
         ),
         swift_toolchain_config.action_config(
             actions = [swift_action_names.PRECOMPILE_C_MODULE],
-            configurators = [
-                lambda prerequisites, args: c_layering_check_configurator(
-                    prerequisites,
-                    args,
-                    strict = False,
-                ),
-            ],
-            not_features = [
-                [SWIFT_FEATURE_LAYERING_CHECK],
-                [SWIFT_FEATURE_SYSTEM_MODULE],
-            ],
-        ),
-        swift_toolchain_config.action_config(
-            actions = [swift_action_names.PRECOMPILE_C_MODULE],
-            configurators = [
-                lambda prerequisites, args: c_layering_check_configurator(
-                    prerequisites,
-                    args,
-                    strict = True,
-                ),
-            ],
+            configurators = [_c_layering_check_configurator],
             features = [SWIFT_FEATURE_LAYERING_CHECK],
             not_features = [SWIFT_FEATURE_SYSTEM_MODULE],
         ),
@@ -789,6 +755,16 @@ def _batch_mode_configurator(prerequisites, args):
     if not _is_wmo_manually_requested(prerequisites.user_compile_flags):
         args.add("-enable-batch-mode")
 
+def _c_layering_check_configurator(prerequisites, args):
+    # We do not enforce layering checks for the Objective-C header generated by
+    # Swift, because we don't have predictable control over the imports that it
+    # generates. Due to modular re-exports (which are especially common among
+    # system frameworks), it may generate an import declaration for a particular
+    # symbol from a different module than the Swift code imported it from.
+    if not prerequisites.is_swift_generated_header:
+        args.add("-Xcc", "-fmodules-strict-decluse")
+    return None
+
 def _clang_search_paths_configurator(prerequisites, args):
     """Adds Clang search paths to the command line."""
     args.add_all(