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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@353695 91177308-0d34-0410-b5e6-96231b3b80d8