[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.

This introduces a change where calls to llvm-readelf with non-ELF object
files that specify --elf-output-style=GNU will now print file summary
information where it previously didn't.

Fixes previous Windows test failure which occured due to JSON escaping
of '\' by not relying on LIT substitution.

Reviewed By: jhenderson

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