Apply upstream clang fix for assertion failure compiling devel/capnproto

Merge commit 48c70c1664aa from llvm git (by Richard Smith):

Extend memset-to-zero optimization to C++11 aggregate functional casts
Aggr{...}.

We previously missed these cases due to not stepping over the additional
AST nodes representing their syntactic form.

Direct commit to stable/11, as both main and stable/13 already have this
fix as part of clang 12.0.1.

Reported by: fernape
PR: 257961