Intellisense support for gcc __attribute__((libcall)) not found

Type: LanguageService

Describe the bug
Certain c libraries use the ((libcall)) attribute in gcc to create interface structs for functions. What it does is it passes the interface base as first argument transparently when calling the function. Like this:

IInterface->SomeFunciton (arg1, arg2, etc);
becomes

IInterface->SomeFunction(IInterface, arg1, arg2, etc);
when constructing the call.

This is not supported in IntelliSense with the current c/c++. It would be a nice-to-have feature when working with these libraries.

• OS and Version: Mac OS X, Windows, Chrome OS
• VS Code Version: Latest
• C/C++ Extension Version: Latest
• Other extensions you installed (and if the issue persists after disabling them):
• A clear and concise description of what the bug is.

To Reproduce

Copy/paste this in vscode and save as test.c:

`#ifdef amigaos4

#ifndef APICALL
#define APICALL attribute((libcall))
#endif

struct IntuitionIFace
{
unsigned int APICALL (*Obtain)(struct IntuitionIFace *Self, const char *arg1, char arg2);
unsigned int APICALL (*Release)(struct IntuitionIFace *Self, char arg3, short arg4);
} iface;

int main() {
struct IntuitionIFace *IIntuition = &iface;
//this below is the call
IIntuition->Obtain("Hello world", 'b');
}
#endif`

It will give am error squiggle at the end of last call.

Expected behavior

No error squiggle. The above compiles and runs with no errors in ppc-amigaos-gcc.

Screenshots

Additional context

Start by reproducing the diagnostic in VS Code with the provided test.c sample and inspect how IntelliSense handles the GCC attribute((libcall)) function-pointer calls. The work is done when the sample call produces no erroneous squiggle while preserving the expected argument behavior.