[libc] wcpcpy implementation (#144802)

Implemented wcpcpy and tests.

---------

Co-authored-by: Sriya Pratipati <sriyap@google.com>

libc/test/src/wchar/CMakeLists.txt

@@ -232,3 +232,13 @@ add_libc_test(
   DEPENDS
     libc.src.wchar.wcscpy
 )
+
+add_libc_test(
+  wcpcpy_test
+  SUITE
+    libc_wchar_unittests
+  SRCS
+    wcpcpy_test.cpp
+  DEPENDS
+    libc.src.wchar.wcpcpy
+)

libc/test/src/wchar/wcpcpy_test.cpp

@@ -0,0 +1,50 @@
+//===-- Unittests for wcpcpy ---------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "hdr/types/wchar_t.h"
+#include "src/wchar/wcpcpy.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcWCPCpyTest, EmptySrc) {
+  // Empty src should lead to empty destination.
+  wchar_t dest[4] = {L'a', L'b', L'c', L'\0'};
+  const wchar_t *src = L"";
+  LIBC_NAMESPACE::wcpcpy(dest, src);
+  ASSERT_TRUE(dest[0] == src[0]);
+  ASSERT_TRUE(dest[0] == L'\0');
+}
+
+TEST(LlvmLibcWCPCpyTest, EmptyDest) {
+  // Empty dest should result in src
+  const wchar_t *src = L"abc";
+  wchar_t dest[4];
+  wchar_t *result = LIBC_NAMESPACE::wcpcpy(dest, src);
+  ASSERT_EQ(dest + 3, result);
+  ASSERT_TRUE(result[0] == L'\0');
+  ASSERT_TRUE(dest[0] == L'a');
+  ASSERT_TRUE(dest[1] == L'b');
+  ASSERT_TRUE(dest[2] == L'c');
+}
+
+TEST(LlvmLibcWCPCpyTest, OffsetDest) {
+  // Offsetting should result in a concatenation.
+  const wchar_t *src = L"abc";
+  wchar_t dest[7];
+  dest[0] = L'x';
+  dest[1] = L'y';
+  dest[2] = L'z';
+  wchar_t *result = LIBC_NAMESPACE::wcpcpy(dest + 3, src);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'y');
+  ASSERT_TRUE(dest[2] == L'z');
+  ASSERT_TRUE(dest[3] == src[0]);
+  ASSERT_TRUE(dest[4] == src[1]);
+  ASSERT_TRUE(dest[5] == src[2]);
+  ASSERT_TRUE(result[0] == L'\0');
+  ASSERT_EQ(dest + 6, result);
+}