[clang][Index] Visit the default parameter arguements in libindex.

Summary:
We are missing the default parmeter arguments when IndexFunctionLocals
is true.

Fixes https://github.com/clangd/clangd/issues/285.

Reviewers: kadircet

Subscribers: kristof.beyls, ilya-biryukov, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74610