Revert "Remove the swift.no_generated_module_map feature" (#1530)

This reverts commit 68b58f7. 

While we have a custom `mixed_language_library` rule that doesnt match
`upstream` we must maintain this feature or we get duplicate module
declaration errors as the mixed language rules depend on this feature
when they create their own modulemap

Without this commit reverted we get:

```sh
bazel-out/ios_sim_arm64-dbg-ios-sim_arm64-min16.0-applebin_ios-ST-2491983d010b/bin/Code/CoreLibraries/Contacts/Aliases/AliasesLegacy/AliasesLegacy.lib_swift_modulemap/_/module.modulemap:1:8: error: redefinition of module 'AliasesLegacy'
    1 | module "AliasesLegacy" {
      |        ^
bazel-out/ios_sim_arm64-dbg-ios-sim_arm64-min16.0-applebin_ios-ST-2491983d010b/bin/Code/CoreLibraries/Contacts/Aliases/AliasesLegacy/AliasesLegacy.lib/AliasesLegacy/module.modulemap:1:8: note: previously defined here
    1 | module "AliasesLegacy" {
      |        ^
1 error generated.
```

Part of: bazelbuild/rules_apple#2729, we can
remove this feature when we upstream the mixed language support in
upstream.

Author: luispadron

swift/internal/BUILD

@@ -156,6 +156,7 @@ bzl_library(
     deps = [
         ":feature_names",
         ":package_specs",
+        ":target_triples",
         "@bazel_skylib//lib:collections",
         "@bazel_skylib//lib:new_sets",
         "@bazel_skylib//rules:common_settings",

swift/internal/compiling.bzl

@@ -48,6 +48,7 @@ load(
     "SWIFT_FEATURE_INDEX_WHILE_BUILDING",
     "SWIFT_FEATURE_MODULAR_INDEXING",
     "SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD",
+    "SWIFT_FEATURE_NO_GENERATED_MODULE_MAP",
     "SWIFT_FEATURE_OPT",
     "SWIFT_FEATURE_OPT_USES_WMO",
     "SWIFT_FEATURE_PROPAGATE_GENERATED_MODULE_MAP",
@@ -724,7 +725,10 @@ to use swift_common.compile(include_dev_srch_paths = ...) instead.\
 
     # If a header and module map were generated for this Swift module, attempt
     # to precompile the explicit module for that header as well.
-    if generated_header_name:
+    if generated_header_name and not is_feature_enabled(
+        feature_configuration = feature_configuration,
+        feature_name = SWIFT_FEATURE_NO_GENERATED_MODULE_MAP,
+    ):
         compilation_context_to_compile = (
             compilation_context_for_explicit_module_compilation(
                 compilation_contexts = [
@@ -1274,7 +1278,10 @@ def _declare_compile_outputs(
     # trap door lets them escape the module redefinition error, with the
     # caveat that certain import scenarios could lead to incorrect behavior
     # because a header can be imported textually instead of modularly.
-    if generated_header:
+    if generated_header and not is_feature_enabled(
+        feature_configuration = feature_configuration,
+        feature_name = SWIFT_FEATURE_NO_GENERATED_MODULE_MAP,
+    ):
         # Collect the names of Clang modules that the module being built
         # directly depends on.
         dependent_module_names = sets.make()

swift/internal/feature_names.bzl

@@ -153,6 +153,11 @@ SWIFT_FEATURE_MODULE_MAP_NO_PRIVATE_HEADERS = (
 # link to any version of the ASAN runtime library.
 SWIFT_FEATURE_NO_ASAN_VERSION_CHECK = "swift.no_asan_version_check"
 
+# If enabled, the compilation action for a library target will not generate a
+# module map for the Objective-C generated header. This feature is ignored if
+# the target is not generating a header.
+SWIFT_FEATURE_NO_GENERATED_MODULE_MAP = "swift.no_generated_module_map"
+
 # If enabled, the parent directory of the generated module map is added to
 # `CcInfo.compilation_context.includes`. This allows `objc_library` to import
 # the Swift module. If you swap this feature between enabled and disabled, and

swift/internal/features.bzl

@@ -33,13 +33,15 @@ load(
     "SWIFT_FEATURE_FILE_PREFIX_MAP",
     "SWIFT_FEATURE_FULL_DEBUG_INFO",
     "SWIFT_FEATURE_INTERNALIZE_AT_LINK",
+    "SWIFT_FEATURE_NO_GENERATED_MODULE_MAP",
     "SWIFT_FEATURE_OBJC_LINK_FLAGS",
     "SWIFT_FEATURE_OPT_USES_WMO",
     "SWIFT_FEATURE_REMAP_XCODE_PATH",
     "SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE",
     "SWIFT_FEATURE__SUPPORTS_V6",
 )
 load(":package_specs.bzl", "label_matches_package_specs")
+load(":target_triples.bzl", "target_triples")
 
 def are_all_features_enabled(feature_configuration, feature_names):
     """Returns `True` if all features are enabled in the feature configuration.
@@ -265,6 +267,12 @@ def default_features_for_toolchain(target_triple):
             SWIFT_FEATURE_REMAP_XCODE_PATH,
         ])
 
+    # Linux specific features
+    if target_triples.unversioned_os(target_triple) == "linux":
+        features.extend([
+            SWIFT_FEATURE_NO_GENERATED_MODULE_MAP,
+        ])
+
     return features
 
 def upcoming_and_experimental_features(feature_configuration):