[llvm-readobj] Add JSONScopedPrinter to llvm-readelf

 Adds JSONScopedPrinter to llvm-readelf. It includes an empty
 JSONELFDumper class which will be used to override any LLVMELFDumper
 methods which utilize startLine() which JSONScopedPrinter cannot
 provide.

 Reviewed By: jhenderson

 Differential Revision: https://reviews.llvm.org/D114225