[Swiftify] Add vsnprintf test

test/Interop/C/swiftify-import/Inputs/module.modulemap

@@ -26,3 +26,7 @@ module CommentsClang {
     header "comments.h"
     export *
 }
+module VsnprintfClang {
+    header "vsnprintf.h"
+    export *
+}

test/Interop/C/swiftify-import/Inputs/vsnprintf.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <stdarg.h>
+#include <stddef.h>
+
+#define __counted_by(x) __attribute__((__counted_by__(x)))
+#define __printf(string_index, first_to_check)                                 \
+  __attribute__((__format__(__printf__, string_index, first_to_check)))
+
+#ifndef __cplusplus
+#define __restrict restrict
+#endif
+
+int vsnprintf(char *__restrict __counted_by(__size) __str, size_t __size,
+              const char *__restrict __format, va_list foo_args) __printf(3, 0);

test/Interop/C/swiftify-import/vsnprintf.swift

@@ -0,0 +1,21 @@
+// REQUIRES: swift_feature_SafeInteropWrappers
+
+// RUN: %target-swift-ide-test -print-module -module-to-print=VsnprintfClang -plugin-path %swift-plugin-dir -I %S/Inputs -source-filename=x -enable-experimental-feature SafeInteropWrappers -Xcc -Wno-nullability-completeness | %FileCheck %s
+
+// swift-ide-test doesn't currently typecheck the macro expansions, so run the compiler as well
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -o %t/Vsnprintf.swiftmodule -I %S/Inputs -enable-experimental-feature SafeInteropWrappers -strict-memory-safety -warnings-as-errors -Xcc -Werror -Xcc -Wno-nullability-completeness %s
+
+// Check that ClangImporter correctly infers and expands @_SwiftifyImport macro for vsnprintf with __counted_by and va_list parameters.
+
+import VsnprintfClang
+
+// CHECK:      /// This is an auto-generated wrapper for safer interop
+// CHECK-NEXT: @_alwaysEmitIntoClient @_disfavoredOverload public func vsnprintf(_ __str: UnsafeMutableBufferPointer<CChar>, _ __format: UnsafePointer<CChar>!, _ foo_args: CVaListPointer) -> Int32
+
+@inlinable
+public func callVsnprintf(_ p: UnsafeMutableBufferPointer<CChar>) {
+    unsafe withVaList([CLongLong(42), CInt(1337)]) { args in
+        _ = unsafe vsnprintf(p.baseAddress!, p.count, "%lld %d", args)
+    }
+}