[llvm-readelf/readobj] - Fix the behavior when a sections is included in two...

[llvm-readelf/readobj] - Fix the behavior when a sections is included in two groups at the same time.

The current behavior was introduced by me in D37567 and it is a bit strange. It prints the
"Error: ...." message to the errs() manually and stops dumping the group section which has this error.
This behavior is consistent with GNU though, but it is very inconsistent with what the regular llvm-readelf
code usually does/prints, so I suggest to change the implementation:

1) Instead of printing "Error: ...." to errs() - just report a warning.
2) Try to continue dumping the section.
3) Merge broken-group.test to group.text.

This is what this patch does.

Differential revision: https://reviews.llvm.org/D84170