Add support for decltype

[julianxhokaxhiu]

Hi,

I was trying to use this tool to generate a struct to import the types of some functions by using decltype but unfortunately the tool seems to not support it.

While trying to use it, I get this error:

# ClangSharpPInvokeGenerator.exe --version
ClangSharp P/Invoke Binding Generator version 14.0.0
  clang version 14.0.0
  clangsharp version 14.0.0
# ClangSharpPInvokeGenerator.exe --file .\struct.h --namespace test --output test.cs
Processing '.\struct.h'
Diagnostics for binding generation of .\struct.h:
    Error (Line 6, Column 23 in ./struct.h): Unsupported type: 'CX_TypeClass_Decltype'.
    Warning (Line 6, Column 23 in ./struct.h): Unsupported type: 'CX_TypeClass_Decltype'. Falling back 'decltype(&ReadFile)'.

Here is an example struct.h file:

#define WIN32_LEAN_AND_MEAN
#include <windows.h>

struct Methods
{
  decltype(&ReadFile) ReadFile;
};

Feel free to ping me if anything else is required.

Thank you in advance,
Julian