Rework the way we find locally-scoped external declarations when we

need them to evaluate redeclarations or call a function that hasn't
already been declared. We now keep a DenseMap of these locally-scoped
declarations so that they are not visible but can be quickly found,
e.g., when we're looking for previous declarations or before we go
ahead and implicitly declare a function that's being called. Fixes
PR3672.

llvm-svn: 65792