[clangd] Add support to extract method for ExtractFunction Tweak

I miss more automatically refactoring functions when working with already running code, so I am making some small addition that I hope help more people.

This works by checking if the function is a method (CXXMethodDecl), then collecting information about the function that the code is being extracted, looking for the declaration if it is out-of-line, creating the declaration if it is necessary and putting the extracted function as a class-method.

This is my first code review request, sorry if I did something wrong.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D122698