[Sema] Discarded statment should be an evaluatable context.

The constexpr evaluator was erroring out because these templates weren't
defined. Despite being used in a discarded statement, we still need to constexpr
evaluate them, which means that we need to instantiate them. Fixes PR37585.

Differential revision: https://reviews.llvm.org/D48322

llvm-svn: 336233