When parsing default function arguments, do not mark any declarations

used in the default function argument as "used". Instead, when we
actually use the default argument, make another pass over the
expression to mark any used declarations as "used" at that point. This
addresses two kinds of related problems:

  1) We were marking some declarations "used" that shouldn't be,
  because we were marking them too eagerly.
  2) We were failing to mark some declarations as "used" when we
  should, if the first time it was instantiated happened to be an
  unevaluated context, we wouldn't mark them again at a later point.

I've also added a potentially-handy visitor class template
EvaluatedExprVisitor, which only visits the potentially-evaluated
subexpressions of an expression. I bet this would have been useful for
noexcept...

Fixes PR5810 and PR8127.

llvm-svn: 113700