Generalize test to also work on Windows

Author: DougGregor

test/Interop/Cxx/extern-c/Inputs/my-memory.h

@@ -0,0 +1,11 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef __SIZE_TYPE__ size_t;
+
+void* memset(void* s, int c, size_t n);
+
+#ifdef __cplusplus
+}
+#endif

test/Interop/Cxx/extern-c/decls.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-emit-silgen(mock-sdk: %clang-importer-sdk) %s -I %S/Inputs -import-bridging-header %S/../../../Inputs/clang-importer-sdk/usr/include/string.h -enable-experimental-cxx-interop | %FileCheck %s
+// RUN: %target-swift-emit-silgen(mock-sdk: %clang-importer-sdk) %s -I %S/Inputs -import-bridging-header %S/Inputs/my-memory.h -enable-experimental-cxx-interop | %FileCheck %s
 
 
 func zerome(ptr: UnsafeMutablePointer<Int>) {