[llvm-readobj] Output valid JSON for GroupSections

[paulkirth]

Summary

The current implementation output the LLVM formatted heading for group
sections, which was not valid JSON. This patch provides two small
customization points that allow the heading to vary between the two
implementations, and another that allows the section members to be
output as valid JSON objects.

[jhenderson]

Please provide before and after examples, and test cases for the fix. Also, please fix the title to use "llvm-readobj" in the commit message/patch description, not simply "readobj".

[paulkirth]

Rebase

• avoid duplication in original patch
• add tests to check the JSON and LLVM differences

[paulkirth]

Rebase to fix patch application

[paulkirth]

Rebase

[paulkirth]

Rebase

[paulkirth]

Rebase.