Clean up uses of unique_ptr for RAII objects. NFC.

- EnterExpressionEvaluationContext allows you to specify whether you
  *actually* want to enter an evaluation context.
- For types that don't allow that, llvm::Optional<Foo> should do the
  same thing as std::unique_ptr<Foo>, but with 100% less heap
  allocations.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@286500 91177308-0d34-0410-b5e6-96231b3b80d8