Fix false linter errors for os.h in Visual Studio/VSCode

[AidanSun05]

The Problem:
When using fmt::file and fmt::output_file with Visual Studio 2019 and Visual Studio Code on Windows, the linters in both yield the following errors because they can't find the required definitions:

However, the actual compile succeeds. This is because a macro (FMT_HAS_INCLUDE) is disabled only when IntelliSense is linting, not when MSVC is compiling (this workaround is from a VS 2015 IntelliSense bug: #842). But disabling this macro in newer versions of VS is no longer necessary and leads to the errors mentioned above.

The Proposed Fix:
Change the definition of FMT_HAS_INCLUDE to:

#if defined(__has_include) &&                                               \
    (!defined(__INTELLISENSE__) || (!FMT_MSC_VER || FMT_MSC_VER > 1900)) && \  // <== (CHANGED THIS)
    (!FMT_ICC_VERSION || FMT_ICC_VERSION >= 1600)
#  define FMT_HAS_INCLUDE(x) __has_include(x)
#else
#  define FMT_HAS_INCLUDE(x) 0
#endif

In addition to making a decision based on the absence/presence of IntelliSense, this revised code will also check if MSVC's version macro is above 1900 (the value corresponding to VS2015). This retains the original workaround while removing the false errors on newer VS versions.

Is this feasible and can this work in all cases? If so, will a PR be accepted? Thanks!

[vitaut]

I can't tell if it will work in all cases but a PR is welcome and if it passes the CI it should be OK.

[AidanSun05]

I have opened: #2370