[DebugInfo/llvm-objdump] - Print "ZERO terminator" for terminator entries when dumping .eh_frame.

A CIE with the Length == 0 is a terminator:
https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html

And GNU objdump recognizes them and prints the following for such entries:

"00000000 ZERO terminator"

This patch teaches llvm-objdump to do the same. I had to update tests to use
"CHECK-NEXT" too.

(Note: it looks perhaps not right that printing is done inside the DebugInfo library,
I'd expect to see the change in the llvm-objdump's code somewhere instead,
but that is how it done atm).

Differential revision: https://reviews.llvm.org/D80476