[Clangd] extract function refactoring design doc

Hi folks,

Here’s the design doc for the newly added Extract Function refactoring I worked on as part of my internship at Google:

https://docs.google.com/document/d/1QecOuIzcBdbkeP9-MStY92EU5vCgchi0zIfqrp27UBk/edit?usp=sharing

Comments and patches are welcome.

Shaurya Gupta
LinkedIn | Github