PInvokeGenerator ignores calling convention attribute on function pointer

[DCNick3]

Repro:
test.cpp:

struct test {
    void (__stdcall* test_cb)(void);
};

Command line (on arch linux): dotnet run -r ubuntu.18.04-x64 -f net5.0 -- -a -target i386-pc-win32 -n test -f test.cpp -o test.cs

Produced output:

namespace test
{
    public unsafe partial struct test
    {
        [NativeTypeName("void (*)() __attribute__((stdcall))")]
        public delegate* unmanaged[Cdecl]<void> test_cb;
    }
}

Expected output:

namespace test
{
    public unsafe partial struct test
    {
        [NativeTypeName("void (*)() __attribute__((stdcall))")]
        public delegate* unmanaged[Stdcall]<void> test_cb;
    }
}

(Notice the ignored calling convention)

I found a codepath where this information was discarded:

https://github.com/microsoft/ClangSharp/blob/73da2e7a8f75a2b789620b0882a55dd66b6ee05c/sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.cs#L1625-L1628

This clause unwraps AttributedType and calls the same function for attributedType.ModifiedType.

attributedType.Handle.FunctionTypeCallingConv is CXCallingConv_X86StdCall, but ModifiedType does not have this information and attributedType.ModifiedType.Handle.FunctionTypeCallingConv is CXCallingConv_C

win32metadata evades this by having Winapi as a default calling convention (--with-callconv *=Winapi) and seemingly all callbacks using this calling convention.