Implement flag enabling Starlark implementation of C++ linking support

Most of the linking code is in Starlark, including a large chunk of LTO support. What's not in Starlark:
- conversion of LibraryToLink into LegacyLinkerInputs (causes a regression)
- conversion of collectLibrariesToLink (causes a regression)
- compilation of LTO artifacts (needs rewrite of C++ compilation support first)

PiperOrigin-RevId: 723497031
Change-Id: I9cd75a9c11de9eb82b277757267bcaeaecfd7034

src/main/java/com/google/devtools/build/lib/rules/cpp/CppConfiguration.java

@@ -1011,4 +1011,13 @@ public boolean getProtoProfile(StarlarkThread thread) throws EvalException {
     CcModule.checkPrivateStarlarkificationAllowlist(thread);
     return cppOptions.protoProfile;
   }
+
+  @StarlarkMethod(
+      name = "experimental_starlark_linking",
+      documented = false,
+      useStarlarkThread = true)
+  public boolean experimentalStarlarkLinking(StarlarkThread thread) throws EvalException {
+    CcModule.checkPrivateStarlarkificationAllowlist(thread);
+    return cppOptions.experimentalStarlarkLinking;
+  }
 }

src/main/java/com/google/devtools/build/lib/rules/cpp/CppOptions.java

@@ -1078,6 +1078,17 @@ public Label getMemProfProfileLabel() {
               + " toolchain() resolution to choose a test runner.")
   public boolean experimentalPlatformCcTest;
 
+  @Option(
+      name = "experimental_starlark_linking",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
+      effectTags = {
+        OptionEffectTag.LOADING_AND_ANALYSIS,
+      },
+      metadataTags = {OptionMetadataTag.EXPERIMENTAL},
+      help = "If enabled, a Starlark version of linking is used.")
+  public boolean experimentalStarlarkLinking;
+
   /** See {@link #targetLibcTopLabel} documentation. * */
   @Override
   public FragmentOptions getNormalized() {

src/main/starlark/builtins_bzl/common/builtin_exec_platforms.bzl

@@ -326,6 +326,7 @@ bazel_fragments["CppOptions"] = fragment(
         "//command_line_option:experimental_platform_cc_test",
         "//command_line_option:experimental_save_feature_state",
         "//command_line_option:experimental_use_llvm_covmap",
+        "//command_line_option:experimental_starlark_linking",
     ],
     outputs = [
         "//command_line_option:crosstool_top",

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

@@ -21,6 +21,8 @@ load(
 )
 load(":common/cc/cc_info.bzl", "CcInfo")
 load(":common/cc/cc_shared_library_hint_info.bzl", "CcSharedLibraryHintInfo")
+load(":common/cc/link/create_linking_context_from_compilation_outputs.bzl", "create_linking_context_from_compilation_outputs")
+load(":common/cc/link/link.bzl", "link")
 load(":common/cc/link/link_build_variables.bzl", "create_link_variables")
 
 cc_common_internal = _builtins.internal.cc_common
@@ -126,7 +128,11 @@ def _link(
     if emit_interface_shared_library == _UNBOUND:
         emit_interface_shared_library = False
 
-    return cc_common_internal.link(
+    if cc_toolchain._cpp_configuration.experimental_starlark_linking():
+        link_func = link
+    else:
+        link_func = cc_common_internal.link
+    return link_func(
         actions = actions,
         name = name,
         feature_configuration = feature_configuration,
@@ -570,7 +576,11 @@ def _create_linking_context_from_compilation_outputs(
     if test_only_target == _UNBOUND:
         test_only_target = False
 
-    return cc_common_internal.create_linking_context_from_compilation_outputs(
+    if cc_toolchain._cpp_configuration.experimental_starlark_linking():
+        linking_func = create_linking_context_from_compilation_outputs
+    else:
+        linking_func = cc_common_internal.create_linking_context_from_compilation_outputs
+    return linking_func(
         actions = actions,
         name = name,
         feature_configuration = feature_configuration,