[Object] Parsing and dumping of SFrame FDEs (#149828)
Also known as Function Description Entries. The entries occupy a
contiguous piece of the section, so the code is mostly straight-forward.
For more information about the SFrame unwind format, see the
[specification](https://sourceware.org/binutils/wiki/sframe) and the
related [RFC](https://discourse.llvm.org/t/rfc-adding-sframe-support-to-llvm/86900).
