Fix PR#62594 : static lambda call operator is not convertible to function pointer on win32

See issue https://github.com/llvm/llvm-project/issues/62594

This code does not work on win32:

  auto lstatic = []()  static  { return 0;  };
  int (*f2)(void) = lstatic;

Since a calling convention such as CC_X86ThisCall can rightly interfere with the implicit pointer to function conversion if erroneously marked on a static function, the fix entails checking the 'static' specifier on the lambda declarator prior to assigning it a calling convention of an non-static member (which pre-c++23 made sense).