Fix bug in code for avoiding dynamic initialization of dllimport globals

When instantiating dllimport variables with dynamic initializers, don't
bail out of Sema::InstantiateVariableInitializer without calling
PopExpressionEvaluationContext().

This was causing a stale object to stay on the ExprEvalContexts stack,
causing subsequent calls to getCurrentMangleNumberContext() to fail,
resulting in incorrect numbering of static locals (and probably other
broken things).