[c-interop][test] Cover default argument values with @_extern

Author: kateinoigakukun

test/IRGen/extern_c.swift

@@ -5,6 +5,11 @@ func test() {
   explicit_extern_c()
   // CHECK: call void @implicit_extern_c()
   implicit_extern_c()
+  // CHECK: [[DEFAULT_ARG:%[0-9]+]] = call swiftcc i32 @"$s8extern_c17default_arg_valueyys5Int32VFfA_"()
+  // CHECK: call void @default_arg_value(i32 [[DEFAULT_ARG]])
+  default_arg_value()
+  // CHECK: call void @default_arg_value(i32 24)
+  default_arg_value(24)
 }
 
 test()
@@ -14,3 +19,6 @@ test()
 
 // CHECK: declare void @implicit_extern_c()
 @_extern(c) func implicit_extern_c()
+
+// CHECK: declare void @default_arg_value(i32)
+@_extern(c) func default_arg_value(_: Int32 = 42)

test/attr/attr_extern.swift

@@ -115,6 +115,11 @@ func nonCParamTypesWasm(_: Int, _: NonC)
 @_extern(wasm, module: "non-c", name: "param_mixed")
 func nonCParamTypesMixed(_: Int, _: NonC) // expected-error {{'NonC' cannot be represented in C}}
 
+@_extern(c)
+func defaultArgValue_C(_: Int = 42)
+
+@_extern(wasm, module: "", name: "")
+func defaultArgValue_Wasm(_: Int = 24)
 
 @_extern(c)
 func asyncFuncC() async // expected-error {{async functions cannot be represented in C}}