[clangd] Include refs of base method in refs for derived method.

Addresses https://github.com/clangd/clangd/issues/881

Includes refs of base class method in refs of derived class method.
Previously we reported base class method's refs only for decl of derived
class method. Ideally this should work for all usages of derived class method.

Related patch:
https://github.com/llvm/llvm-project/commit/fbeff2ec2bc6e44b92931207b0063f83ff7a3b3a.

Differential Revision: https://reviews.llvm.org/D111039