[compiler-rt] Make the MSAN wmemset intercepter call wmemset instead of memset. Fixes PR 21579

[EricWF]

Summary

Exactly what the title says. I've tested this change against the libc++ test failures and it solves all of them. The check-msan rule also still passes.
I'm not sure why it called memset originally.

I can add tests if requested but currently there are no tests involving wide chars and they are a c++11 features.

[eugenis]

Ouch.
Thanks a lot.
There are some wchar_t tests in lib/msan/tests/msan_test.cc, please add one.

[EricWF]

Added a test case. Could somebody just double check it looks sane? I'm not quite comfortable with the msan test suite.

[EricWF]

Woops, The last test case was wrong. Correct test case attached. Sorry for the noise.

[eugenis]

Looks great.

lib/msan/msan_interceptors.cc

 INTERCEPTOR(wchar_t *, wmemset, wchar_t *s, wchar_t c, SIZE_T n) {
   CHECK(MEM_IS_APP(s));
   ENSURE_MSAN_INITED();
-  wchar_t *res = (wchar_t *)REAL(memset)(s, c, n * sizeof(wchar_t));
+  wchar_t *res = REAL(wmemset)(s, c, n);
   __msan_unpoison(s, n * sizeof(wchar_t));
   return res;
 }

lib/msan/tests/msan_test.cc

   EXPECT_POISONED(buff[2]);
 }
 
+TEST(MemorySanitizer, wmemset) {
+    wchar_t x[25];
+    break_optimization(x);
+    EXPECT_POISONED(x[0]);
+    wmemset(x, L'A', 10);
+    EXPECT_EQ(x[0], L'A');
+    EXPECT_EQ(x[9], L'A');
+    EXPECT_POISONED(x[10]);
+}
+
 TEST(MemorySanitizer, mbtowc) {
   const char *x = "abc";
   wchar_t wx;