Fix an assertion when ending a function definition.

The bug is that ExprCleanupObjects isn't always empty
in a fresh evaluation context.  New evaluation contexts just
track the current depth of the stack.

The assertion will misfire whenever we finish processing
a function body inside an expression that contained an earlier
block literal with non-trivial captures.  That's actually
a lot less likely than you'd think, though, because it has
to be a real function declaration, not just another block.
Mixed block/lambda code would work, as would a template
instantiation or a local class definition.

The code works correctly if the assertion is disabled.

rdar://16356628