[AST] default implementation is possible for non-member functions in C++20.

Summary:
Make RAV not visit the default function decl by default.
Also update some stale comments on FunctionDecl::isDefault.

Fixes https://github.com/clangd/clangd/issues/383

Reviewers: sammccall, rsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80288