[AST] Fix RecursiveASTVisitor visiting implicit constructor initializers.

Summary: RecursiveASTVisitor was visiting implcit constructor initializers. This caused semantic highlighting in clangd to emit error logs. Fixes this by checking if the constructor is written or if the visitor should visit implicit decls.

Reviewers: hokein, ilya-biryukov

Subscribers: kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65735

llvm-svn: 367839