Rework the way we track which declarations are "used" during

compilation, and (hopefully) introduce RAII objects for changing the
"potentially evaluated" state at all of the necessary places within
Sema and Parser. Other changes:

  - Set the unevaluated/potentially-evaluated context appropriately
    during template instantiation.
  - We now recognize three different states while parsing or
    instantiating expressions: unevaluated, potentially evaluated, and
    potentially potentially evaluated (for C++'s typeid).
  - When we're in a potentially potentially-evaluated context, queue
    up MarkDeclarationReferenced calls in a stack. For C++ typeid
    expressions that are potentially evaluated, we will play back
    these MarkDeclarationReferenced calls when we exit the
    corresponding potentially potentially-evaluated context.
  - Non-type template arguments are now parsed as constant
    expressions, so they are not potentially-evaluated.

llvm-svn: 73899