[AST] Treat semantic form of InitListExpr as implicit code in traversals

Summary:
In particular, do not traverse the semantic form if shouldVisitImplicitCode()
returns false.

This simplifies the common case of traversals, avoiding the need to
worry about some expressions being traversed twice.

No tests break after the change, the change would allow to simplify at
least one of the usages, i.e. r366070 which had to handle this in
clangd.

Reviewers: gribozavr

Reviewed By: gribozavr

Subscribers: kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64762

llvm-svn: 366672