Suppress formattable hides the global definition warning

[evanwporter]

This fixes the warning that is received on MSVC v17.3.6 with the compiler flag /W4. The warning is as follows: declaration of 'formattable' hides global declaration. It appears on every file where fmt/core.h is included.

I renamed it to is_formattable but this can be changed since it seems like is_formattable is used in other places as well.

EDIT: After looking through other PRs, namely #4377, I thought it may be preferred (and less breaking) to suppress the warnings via macros rather than change existing code.

[vitaut]

Thanks for the PR but we try to avoid cluttering the code with compiler-specific pragmas.

[evanwporter]

@vitaut

Idk if its possible for me to reopen but I renamed formattable to is_it_formattable. This will get rid of MSVC warning 4459. At the same time it avoids conflicts with the already named variable is_formattable

[dinomight]

@evanwporter See #4354 . The issue should go away in newer compilers, even if the fix for older compilers isn't significant.