[libc] add mempcpy to bazel overlay (#75383)

We'd like to begin overlaying mempcpy.

Author: nickdesaulniers

utils/bazel/llvm-project-overlay/libc/BUILD.bazel

@@ -2274,6 +2274,19 @@ libc_function(
     ],
 )
 
+libc_function(
+    name = "mempcpy",
+    srcs = ["src/string/mempcpy.cpp"],
+    hdrs = ["src/string/mempcpy.h"],
+    copts = ["-mllvm --tail-merge-threshold=0"],
+    features = no_sanitize_features,
+    weak = True,
+    deps = [
+        ":__support_common",
+        ":string_memory_utils",
+    ],
+)
+
 libc_function(
     name = "bcopy",
     srcs = ["src/string/bcopy.cpp"],

utils/bazel/llvm-project-overlay/libc/test/src/string/BUILD.bazel

@@ -135,6 +135,14 @@ libc_test(
     deps = [":memory_check_utils"],
 )
 
+libc_test(
+    name = "mempcpy_test",
+    srcs = ["mempcpy_test.cpp"],
+    libc_function_deps = [
+        "//libc:mempcpy",
+    ],
+)
+
 libc_test(
     name = "memset_test",
     srcs = ["memset_test.cpp"],