Various cleanups: Store properly-typed function pointers in winapisupp.cpp

[StephanTLavavej]

winapisupp.cpp: Use separate function pointers with proper types, instead of enumerators into an array of PVOID.

This is within an unnamed namespace (after #2841 dramatically simplified the code), so we can freely modify this machinery.

Originally, the function pointer table's ordering seemed really important to us, and we were scared about adding/removing/reordering entries. #2841 showed that those fears were unfounded, but it was changing a ton of code so I didn't eliminate the table entirely at that time.

The table wasn't buying us anything - we were able to properly size it via eMaxKernel32Function, but we still had to mention every function to manually form the eMeow enumerators. By introducing DEFINEFUNCTIONPOINTER, we can spend the same number of lines to form separate global variables, which will still consume the same amount of memory as the array (they're just pointers, they pack perfectly either way).

I'm naming these function pointers __KERNEL32Function_##fn_name with an underscore in the middle for clarity, e.g. __KERNEL32Function_GetTempPath2W. I thought about omitting Function but it seems reasonable here. The advantage of this technique is that every function pointer can be given the correct type decltype(&fn_name). We need to adjust how we cast the return value of GetProcAddress(), but then we don't need to cast when reading the variable later.

Note that IFDYNAMICGETCACHEDFUNCTION still forms const auto pf##fn_name to defend against modifying the cache unintentionally, and to provide a nice name pfMeow for the code that follows its usage.

Finally, I'm changing all of the macro parameter names to fn_name for consistency.

@frederick-vs-ja noticed an opportunity to simplify if (T meow = init; meow) to if (T meow = init); I also found a couple of lines in stl/src/tzdb.cpp that could benefit.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.