SwiftShims: memcmp should accept optional pointers on Darwin

Darwin defines memcmp with optional pointers. Update SwiftShims to
define it to the same type to avoid deserialization failures where we
get one over the other and the types don't match anymore.

rdar://140596571

Author: xymus

stdlib/public/SwiftShims/swift/shims/LibcShims.h

@@ -60,8 +60,11 @@ static inline __swift_size_t _swift_stdlib_strlen_unsigned(const unsigned char *
 SWIFT_READONLY
 static inline int _swift_stdlib_memcmp(const void *s1, const void *s2,
                                        __swift_size_t n) {
+#if defined(__APPLE__)
+  // Darwin defines memcmp with optional pointers, preserve the same type here.
+  extern int memcmp(const void * _Nullable, const void * _Nullable, __swift_size_t);
 // FIXME: Is there a way to identify Glibc specifically?
-#if (defined(__gnu_linux__) || defined(__ANDROID__)) && !defined(__musl__)
+#elif (defined(__gnu_linux__) || defined(__ANDROID__)) && !defined(__musl__)
   extern int memcmp(const void * _Nonnull, const void * _Nonnull, __swift_size_t);
 #else
   extern int memcmp(const void * _Null_unspecified, const void * _Null_unspecified, __swift_size_t);

validation-test/ClangImporter/memcmp-definitions.swift

@@ -1,4 +1,5 @@
-/// rdar://69876253
+/// Darwin's memcmp accepts nullable pointers, make sure the SwiftShims one
+/// preserves the same type.
 // REQUIRES: VENDOR=apple
 // RUN: %target-build-swift %s -o %t.out
 
@@ -9,4 +10,6 @@ func foo () {
   let a = UnsafeMutableRawPointer.allocate(byteCount: 4, alignment: 4)
   let b = UnsafeMutableRawPointer.allocate(byteCount: 4, alignment: 4)
   memcmp(a, b, 4)
+
+  memcmp(nil, nil, 0)
 }