[clangd] Change constness of parameters to findExplicitRefs

Summary:
Recursive AST requires non-const ast nodes, but it doesn't really
mutate them. In addition to that, in clangd we mostly have const ast nodes. So
it makes sense to move the const_cast into callee rather than having it at every
caller in the future.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68027

llvm-svn: 372888