finished mbstowcs implementation

Author: Sriya Pratipati

libc/src/wchar/mbstowcs.cpp

@@ -23,7 +23,20 @@ LLVM_LIBC_FUNCTION(int, mbstowcs,
                     size_t n)) {
   static internal::mbstate internal_mbstate;
   internal::StringConverter<char8_t> str_conv(
-      reinterpret_cast<const char8_t *>(pwcs), &internal_mbstate, n);
+      reinterpret_cast<const char8_t *>(s), &internal_mbstate, n);
+  int dst_idx = 0;
+  ErrorOr<char32_t> converted = str_conv.popUTF32();
+  while (converted.has_value()) {
+    if (pwcs != nullptr)
+      pwcs[dst_idx] = converted.value();
+    dst_idx++;
+    converted = str_conv.popUTF32();
+  }
+  if (converted.error() == -1) // if we hit conversion limit
+    return dst_idx;
+
+  libc_errno = converted.error();
+  return -1;
 }
 
 } // namespace LIBC_NAMESPACE_DECL