precompile and cache Carbon prelude (#7432)

Refactors the link driver to automatically compile and cache the carbon
prelude for use in linking.

Implements a `carbon_library` rule for compiling the Core library
dependencies in the examples.

toolchain/driver/link_options.cpp

@@ -27,8 +27,8 @@ system mixes object files and linker flags.
 
 }  // namespace
 
-auto LinkOptions::BuildForLinkSubcommand(CommandLine::CommandBuilder& b)
-    -> void {
+auto LinkOptions::BuildForLinkSubcommand(CommandLine::CommandBuilder& b,
+                                         CodegenOptions* cg_options) -> void {
   b.AddStringPositionalArg(
       {
           .name = "OBJECT_FILE",
@@ -56,14 +56,27 @@ legacy parsing logic.
       },
       [&](auto& arg_b) { arg_b.Set(&output_filename); });
 
-  BuildSharedOptions(b, this);
+  b.AddFlag(
+      {
+          .name = "link_prelude_files",
+          .help = R"""(
+Automatically include the Carbon prelude files in the link.
+)""",
+      },
+      [&](auto& arg_b) {
+        arg_b.Default(true);
+        arg_b.Set(&link_prelude_files);
+      });
 
-  codegen_options = std::make_shared<CodegenOptions>();
-  codegen_options->Build(b);
+  BuildSharedOptions(b, this);
+  cg_options->Build(b);
+  codegen_options = cg_options;
 }
 
-auto LinkOptions::BuildForBuildSubcommand(CommandLine::CommandBuilder& b)
-    -> void {
+auto LinkOptions::BuildForBuildSubcommand(CommandLine::CommandBuilder& b,
+                                          CodegenOptions* cg_options) -> void {
+  codegen_options = cg_options;
+
   b.AddStringOption(
       {
           .name = "output",
@@ -77,6 +90,9 @@ name of the first provided input file.
       [&](auto& arg_b) { arg_b.Set(&output_filename); });
 
   BuildSharedOptions(b, this);
+  // The prelude files have already been built and are part of the input binary
+  // set, so will already be linked through that path.
+  link_prelude_files = false;
 }
 
 }  // namespace Carbon