[cxx-interop] Do not crash for `static operator()`
This fixes an assertion failure:
```
Assertion failed: (isInstance() && "No 'this' for static methods!"), function getThisType, file DeclCXX.cpp, line 2636.
```
Clang changed it's handling of member call expressions in https://github.com/llvm/llvm-project/commit/af4751738db89a142a8880c782d12d4201b222a8 causing the assertion to fail when synthesizing a forwarding function declaration for `static operator()`.
rdar://133257179
