This is unneeded in almost all circumstances. We only return an APValue
back to clang when the evaluation is finished, and that is always done
by an EvalEmitter - which has its own implementation of the Ret
instructions.
The global scope we create when evaluating expressions might free some
of the dynamic memory allocations, so we can't check for memory leaks
before destroying it.
I started out by adding a new pointer type for blocks, and I was fully
prepared to compile their AST to bytecode and later call them.
... then I found out that the current interpreter doesn't support
calling blocks at all. So we reuse `Function` to support sources other
than `FunctionDecl`s and classify `BlockPointerType` as `PT_FnPtr`.
This happens for enum types with bool parent types. isBooleanType()
returns false for them however.
The previous version did the same thing by re-classifying the enum
integer type, but that breaks with forward-declared enums.
