Fix misleading msvc macro and unreachable return in swap function

[rilysh]

Hello,
This PR addresses the following issues.

1. In several files, the swap(...) function has used MSC_VER instead of _MSC_VER which will lead them to fall back to the manual implementation of the byte swap(...) function.
2. If the compiler has built-in support for byte swapping then it generally will create a warning about the use of two returns, where the last one is unreachable since the built-in has already been provided. Using #else for the manual implementation (e.g. If the compiler doesn't have a built-in byte swap function), then the warning can be avoided.