[clang] [clang][HIP] Normalize __cdecl on abstract function types to the device default CC on amdgcnspirv (PR #210882)

[Mariya Podchishchaeva via cfe-commits]

================
@@ -8349,6 +8349,19 @@ static bool handleFunctionTypeAttr(TypeProcessingState &state, ParsedAttr &attr,
     return true;
 
   const FunctionType *fn = unwrapped.get();
+
+  // On amdgcnspirv, __cdecl on an abstract function type has no device-side
+  // equivalent, so normalize it to the device default CC to match an
+  // unadorned function type. Skip named decls and variadic types.
+  if (CC == CC_C && S.getLangOpts().CUDAIsDevice &&
----------------
Fznamznon wrote:

SYCL is running into similar problems with Windows headers and we've been thinking that perhaps SPIR-V could just use CC_C as a default calling convention on AST level and just map it to spir_func on codegen stage.

There is another problem Windows headers provoke when compiled with SPIR-V target, consider
```
class A {
public:
  virtual void  __attribute__((thiscall)) boo() {}
};

class B : public A {
  virtual void boo() override {}
};
```
It provokes errors when compiled for SPIR targets.
The problem here is that `boo` has default calling convention which is CC_SpirFunction and `foo` has thiscall calling convetion which is transformed to `CC_C` in most cases. This can be solved if SPIR-V targets use `CC_C` calling convention on AST level.
cc @tahonermann  @schittir 

https://github.com/llvm/llvm-project/pull/210882