[libc] fix msan failure in mempcpy_test (llvm#75532)

Internal builds of the unittests with msan flagged mempcpy_test.

    ==6862==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x55e34d7d734a in length
llvm-project/libc/src/__support/CPP/string_view.h:41:11
#1 0x55e34d7d734a in string_view
llvm-project/libc/src/__support/CPP/string_view.h:71:24
rust-lang#2 0x55e34d7d734a in
__llvm_libc_9999_0_0_git::testing::Test::testStrEq(char const*, char
const*, char const*, char const*,
__llvm_libc_9999_0_0_git::testing::internal::Location)
llvm-project/libc/test/UnitTest/LibcTest.cpp:284:13
rust-lang#3 0x55e34d7d4e09 in LlvmLibcMempcpyTest_Simple::Run()
llvm-project/libc/test/src/string/mempcpy_test.cpp:20:3
rust-lang#4 0x55e34d7d6dff in
__llvm_libc_9999_0_0_git::testing::Test::runTests(char const*)
llvm-project/libc/test/UnitTest/LibcTest.cpp:133:8
rust-lang#5 0x55e34d7d86e0 in main
llvm-project/libc/test/UnitTest/LibcTestMain.cpp:21:10

SUMMARY: MemorySanitizer: use-of-uninitialized-value
llvm-project/libc/src/__support/CPP/string_view.h:41:11 in length

What's going on here is that mempcpy_test.cpp's Simple test is using
ASSERT_STREQ with a partially initialized char array. ASSERT_STREQ calls
Test::testStrEq which constructs a cpp:string_view. That constructor
calls the
private method cpp::string_view::length. When built with msan, the loop
is
transformed into multi-byte access, which then fails upon access.

I took a look at libc++'s __constexpr_strlen which just calls
__builtin_strlen(). Replacing the implementation of
cpp::string_view::length
with a call to __builtin_strlen() may still result in out of bounds
access when
the test is built with msan.

It's not safe to use ASSERT_STREQ with a partially initialized array.
Initialize the whole array so that the test passes.

libc/test/src/string/mempcpy_test.cpp

@@ -14,7 +14,7 @@
 // mempcpy behavior (returning the end of what was copied).
 TEST(LlvmLibcMempcpyTest, Simple) {
   const char *src = "12345";
-  char dest[10];
+  char dest[10] = {};
   void *result = LIBC_NAMESPACE::mempcpy(dest, src, 6);
   ASSERT_EQ(static_cast<char *>(result), dest + 6);
   ASSERT_STREQ(src, dest);