[JITLink] Fix splitBlock if there are symbols span across the boundary

Fix `splitBlock` so that it can handle the case when the block being
split has symbols span across the split boundary. This is an error
case in general but for EHFrame splitting on macho platforms, there is an
anonymous symbol that marks the entire block. Current implementation
will leave a symbol that is out of bound of the underlying block. Fix
the problem by dropping such symbols when the block is split.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D113912