Request: Honor `SupportedOSPlatformAttribute` when emitting `.g.c` file.

Would be nice if this would work…

    public static Exports
    {
        [SupportedOSPlatform("windows")]
        [UnmanagedCallersOnly(EntryPoint = "GetPluginFactory")]
        public static IntPtr GetPluginFactory_Win()
        {  ...  }

        [SupportedOSPlatform("linux")]
        [UnmanagedCallersOnly(EntryPoint = "GetPluginFactory")]
        public static IntPtr GetPluginFactory_Lnx()
        {  ...  }

Currently (v1.0.23) there is an (C/C++) compiler error that indicates both variations are being emitted in the generated .c file. Windows error C2084: function 'intptr_t GetPluginFactory(void)' already has a body Linux error G94CD4D9A: redefinition of 'GetPluginFactory'

[AaronRobinsonMSFT]

@obiwanjacobi Published https://www.nuget.org/packages/DNNE/1.0.24

[jkoritzinsky]

The canonical implementation for the platform attributes is as follows:

The attributes (Supported/UnsupportedOSPlatformAttribute) can each be applied at one of the following levels (generalized for simplicity):

• Assembly
• Module
• Type declaration
• Member declaration

At the highest level that any of the attributes are applied, either SupportedOSPlatformAttribute or UnsupportedOSPlatformAttribute may be applied. This defines an include/exclude list. Any deeper nested level (lower in the list) can apply UnsupportedOSPlatformAttribute to slim down the cases where the most global SupportedOSPlatformAttribute applies, or vice versa. Nested scopes cannot expand the include-list or exclude-list, only slim it down.

Here’s a link to the design doc for the attributes: https://github.com/dotnet/designs/blob/main/accepted/2020/platform-exclusion/platform-exclusion.md#design