initial implementation of the darwin_objc unstable feature

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1237,6 +1237,7 @@ unsafe extern "C" {
     pub(crate) safe fn LLVMSetGlobalConstant(GlobalVar: &Value, IsConstant: Bool);
     pub(crate) safe fn LLVMSetTailCall(CallInst: &Value, IsTailCall: Bool);
     pub(crate) safe fn LLVMSetTailCallKind(CallInst: &Value, kind: TailCallKind);
+    pub(crate) safe fn LLVMSetExternallyInitialized(GlobalVar: &Value, IsExtInit: Bool);
 
     // Operations on attributes
     pub(crate) fn LLVMCreateStringAttribute(

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -258,6 +258,10 @@ pub(crate) fn set_alignment(llglobal: &Value, align: Align) {
     }
 }
 
+pub(crate) fn set_externally_initialized(llglobal: &Value, is_ext_init: bool) {
+    LLVMSetExternallyInitialized(llglobal, is_ext_init.to_llvm_bool());
+}
+
 /// Get the `name`d comdat from `llmod` and assign it to `llglobal`.
 ///
 /// Inserts the comdat into `llmod` if it does not exist.