[clang][pr55896]:co_yield/co_await thread-safety

co_await and co_yield are represented by (classes derived from)
CoroutineSuspendExpr.  That has a number of child nodes, not all of
which are used for code-generation.  In particular the operand is
represented multiple times, and, like the problem with co_return
(55406) it must only be emitted in the CFG exactly once.  The operand
also appears inside OpaqueValueExprs, but that's ok.

This adds a visitor for SuspendExprs to emit the required children in
the correct order.  Note that this CFG is pre-coro xform.  We don't
have initial or final suspend points.

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D127236