[JITLink][ELF] Add error checks to the ELFLinkGraphBuilder.

We now check for:
1. Duplicate definitions of the same section name with different permissions.
2. Symbols whose size extends past the end of the containing block.

It's not clear to me whether either of these is strictly prohibited by the ELF
spec, but they seem pathalogical and JITLink doesn't currently handle them. For
now we'll make them recoverable errors so that if we do encounter these in the
wild we can report the issue(s).