Improve DateTime{Offset}.ParseExact{Multiple} performance for RFC1123 ("r") (dotnet...

Improve DateTime{Offset}.ParseExact{Multiple} performance for RFC1123 ("r") (dotnet/coreclr#18771)

* Improve DateTime{Offset}.ParseExact{Multiple} performance for RFC1123 ("r")

Significantly improves the performance by porting and adapting the Utf8Parser code from corefx.  This optimizes for the (default) case of a DateTimeStyles.None; specifying any other style falls back to the normal parsing support, as that requires handling things such as arbitrary whitespace anywhere in the string.

* Address PR feedback

Commit migrated from https://github.com/dotnet/coreclr/commit/9f9a37d04460559c3f91927f1297c8195411a0a3