First part of P0482 - Establish that char8_t is an integral type, and that numeric_limits<char8_t> is valid and sane. (second try)

llvm-svn: 347930
diff --git a/libcxx/include/limits b/libcxx/include/limits
index f530507..5b75f4a9 100644
--- a/libcxx/include/limits
+++ b/libcxx/include/limits
@@ -82,6 +82,7 @@
 template<> class numeric_limits<cv signed char>;
 template<> class numeric_limits<cv unsigned char>;
 template<> class numeric_limits<cv wchar_t>;
+template<> class numeric_limits<cv char8_t>; // C++20
 template<> class numeric_limits<cv char16_t>;
 template<> class numeric_limits<cv char32_t>;