[wasm] Add support for __builtin_va_list type mapping (#84029)

Cover all va_list family of types just aliased by __builtin_va_list, including __isoc_va_list from wasi-libc. This enables proper C interop for variable argument functions on WASI targets.

Close https://github.com/swiftlang/swift/issues/72398

test/ClangImporter/Inputs/custom-modules/CVarArgs.h

@@ -0,0 +1,7 @@
+#include <stdarg.h>
+extern void takeVaList(va_list args);
+
+extern void takeVaList2(__builtin_va_list args);
+
+typedef __builtin_va_list my_builtin_va_list_alias;
+extern void takeVaList3(my_builtin_va_list_alias args);

test/ClangImporter/Inputs/custom-modules/module.modulemap

@@ -283,3 +283,8 @@ module Aliases {
 module RetroactiveVersioning {
   header "versioning.h"
 }
+
+module CVarArgs {
+  header "CVarArgs.h"
+  export *
+}

test/ClangImporter/ctypes_valist_wasm.swift

@@ -0,0 +1,18 @@
+// RUN: %swift-frontend -target wasm32-unknown-wasip1 -parse-stdlib -module-name Swift -I %S/Inputs/custom-modules -typecheck %s
+
+// REQUIRES: CODEGENERATOR=WebAssembly
+
+import CVarArgs
+
+/// ===== Minimal stdlib definitions =====
+typealias Void = ()
+struct CVaListPointer {}
+// Optional definition is required for isOptional check in ClangImporter
+enum Optional<T> {}
+/// ======================================
+
+func testVaList() {
+  let _: (CVaListPointer) -> Void = CVarArgs.takeVaList
+  let _: (CVaListPointer) -> Void = CVarArgs.takeVaList2
+  let _: (CVaListPointer) -> Void = CVarArgs.takeVaList3
+}