[K/N] Introduce RuntimeInitializer type in the code generation

Kotlin/Native runtime has a concept of `Initializer` (see the type
declaration in `Runtime.h`).

The code generator generates functions of this type. But the
implementation is convoluted, with many different things called "init"
or "initializer".

This commit encapsulates `Initializer` functions to the new
`RuntimeInitializer` type in the code generator, making the
implementation easier to reason about.

^KT-74764

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/llvm/ContextUtils.kt

@@ -641,3 +641,9 @@ internal class CodegenLlvmHelpers(private val generationState: NativeGenerationS
 }
 
 class IrStaticInitializer(val konanLibrary: KotlinLibrary?, val initializer: LlvmCallable)
+
+/**
+ * Function of the [CodeGeneratorVisitor.kInitFuncType] type (aka `Initializer` in `Runtime.h`).
+ */
+@JvmInline
+value class RuntimeInitializer(val llvmCallable: LlvmCallable)

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/llvm/IrToBitcode.kt

@@ -445,9 +445,8 @@ internal class CodeGeneratorVisitor(
     val FILE_NOT_INITIALIZED = 0
     val FILE_INITIALIZED = 2
 
-    private fun createInitBody(state: ScopeInitializersGenerationState): LlvmCallable {
-        val initFunctionProto = kInitFuncType.toProto("", null, LLVMLinkage.LLVMPrivateLinkage)
-        return generateFunction(codegen, initFunctionProto) {
+    private fun createInitBody(state: ScopeInitializersGenerationState): RuntimeInitializer {
+        return generateRuntimeInitializer {
             using(FunctionScope(function, this)) {
                 val bbInit = basicBlock("init", null)
                 val bbLocalInit = basicBlock("local_init", null)
@@ -510,10 +509,16 @@ internal class CodeGeneratorVisitor(
         }
     }
 
+    private fun generateRuntimeInitializer(block: FunctionGenerationContext.() -> Unit): RuntimeInitializer {
+        val initFunctionProto = kInitFuncType.toProto("", null, LLVMLinkage.LLVMPrivateLinkage)
+        return RuntimeInitializer(generateFunction(codegen, initFunctionProto, code = block))
+    }
+
     //-------------------------------------------------------------------------//
     // Creates static struct InitNode $nodeName = {$initName, NULL};
 
-    private fun createInitNode(initFunction: LlvmCallable): LLVMValueRef {
+    private fun createInitNode(runtimeInitializer: RuntimeInitializer): LLVMValueRef {
+        val initFunction = runtimeInitializer.llvmCallable
         val nextInitNode = LLVMConstNull(pointerType(kNodeInitType))
         val argList = cValuesOf(initFunction.toConstPointer().llvm, nextInitNode)
         // Create static object of class InitNode.