Revert "Fix wcpncpy() return value; add test." (#146752)

This reverts commit 988876c.
Was intended to be a PR

Author: michaelrj-google

libc/src/wchar/wcpncpy.cpp

@@ -28,9 +28,8 @@ LLVM_LIBC_FUNCTION(wchar_t *, wcpncpy,
   for (i = 0; i < n && s2[i] != '\0'; ++i)
     s1[i] = s2[i];
   // When n>strlen(src), n-strlen(src) \0 are appended.
-  for (size_t j = i; j < n; ++j)
-    s1[j] = L'\0';
-  // ...but our result points to the first \0 (if any).
+  for (; i < n; ++i)
+    s1[i] = L'\0';
   return s1 + i;
 }
 

libc/test/src/wchar/wcpncpy_test.cpp

@@ -75,15 +75,6 @@ TEST(LlvmLibcWCPNCpyTest, CopyTwoWithNull) {
   ASSERT_EQ(dest + 2, res);
 }
 
-TEST(LlvmLibcWCPNCpyTest, CopyAndFill) {
-  wchar_t dest[] = {L'a', L'b', L'c'};
-  wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, L"x", 3);
-  ASSERT_TRUE(dest[0] == L'x');
-  ASSERT_TRUE(dest[1] == L'\0');
-  ASSERT_TRUE(dest[2] == L'\0');
-  ASSERT_EQ(dest + 1, res);
-}
-
 #if defined(LIBC_ADD_NULL_CHECKS) && !defined(LIBC_HAS_SANITIZER)
 TEST(LlvmLibcWCPNCpyTest, NullptrCrash) {
   // Passing in a nullptr should crash the program.