[llvm-readobj] Rename JSON Flag fields to be more consistent

[paulkirth]

Summary

Today the JSON uses Value and RawValue when printing Flags, when really
the Value field is always the name of an Enum variant, and RawValue is its
underlying numeric value. Similarly, we rename the RawFlags key to Value,
to match the new scheme. This also allows JSON parsing to use consistent logic
for Flag types.

[paulkirth]

Rebase

[paulkirth]

Rebase

[paulkirth]

Rebase

[paulkirth]

Fix lit test, not updated during a rebase

[paulkirth]

Fix test and rebase

[paulkirth]

Rebase.