[llvm-readobj] Improve JSON output

The current implementation outputs JSON in the following way:

[{'<filename>':{'FileSummary':{},...}}]

Using the filename as a key makes processing the JSON data awkward, and
should be avoided. This patch removes that outer key, since the
'FileSummary' data also includes a 'File' field, and so we lose no data.

Reviewed By: jhenderson, leonardchan

Differential Revision: https://reviews.llvm.org/D134843