Refactor our handling of expression evaluation contexts, so that Sema

Refactor our handling of expression evaluation contexts, so that Sema
maintains a stack of evaluation contexts rather than having the parser
do it. This change made it simpler to track in which contexts
temporaries were created, so that we could...

"Forget" about temporaries created within unevaluated contexts, so
that we don't build a CXXExprWithTemporaries and, therefore, destroy
the integral-constness of our expressions. Fixes PR5609.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@89908 91177308-0d34-0410-b5e6-96231b3b80d8