[clang][Index] Add a knob to index function parameters in declarations

Summary:
Parameters in declarations are useful for clangd, so that we can
provide symbol information for them as well. It also helps clangd to be
consistent whether a function's definition is accessible or not.

Reviewers: hokein, akyrtzi

Subscribers: ilya-biryukov, ioeric, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57949

llvm-svn: 353695