rustc: Set release mode cgus to 16 by default

This commit is the next attempt to enable multiple codegen units by default in
release mode, getting some of those sweet, sweet parallelism wins by running
codegen in parallel. Performance should not be lost due to ThinLTO being on by
default as well.

Closes #45320

src/rustllvm/PassWrapper.cpp

@@ -1182,6 +1182,15 @@ LLVMRustThinLTOPatchDICompileUnit(LLVMModuleRef Mod, DICompileUnit *Unit) {
   MD->addOperand(Unit);
 }
 
+extern "C" void
+LLVMRustThinLTORemoveAvailableExternally(LLVMModuleRef Mod) {
+  Module *M = unwrap(Mod);
+  for (Function &F : M->functions()) {
+    if (F.hasAvailableExternallyLinkage())
+      F.deleteBody();
+  }
+}
+
 #else
 
 extern "C" bool
@@ -1272,4 +1281,10 @@ extern "C" void
 LLVMRustThinLTOPatchDICompileUnit(LLVMModuleRef Mod) {
   report_fatal_error("ThinLTO not available");
 }
+
+extern "C" void
+LLVMRustThinLTORemoveAvailableExternally(LLVMModuleRef Mod) {
+  report_fatal_error("ThinLTO not available");
+}
+
 #endif // LLVM_VERSION_GE(4, 0)