[clangd] DefineOutline won't copy virtual specifiers on methods

Summary:
The define out of line refactor tool previously would copy the `virtual`, `override` and `final` specifier into the out of line method definition.
This results in malformed code as those specifiers aren't allowed outside the class definition.

Reviewers: hokein, kadircet

Reviewed By: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D75429