[dwarf] Unify unknown dwarf enum formatting code

Summary:
We have had at least three pieces of code (in DWARFAbbreviationDeclaration,
DWARFAcceleratorTable and DWARFDie) that have hand-rolled support for
dumping unknown dwarf enum values. While not terrible, they are a bit
distracting and enable small differences to creep in (Unknown_ffff vs.
Unknown_0xffff). I ended up needing to add a fourth place
(DWARFVerifier), so it seems it would be a good time to centralize.

This patch creates an alternative to the XXXString dumping functions in
the BinaryFormat library, which formats an unknown value as
DW_TYPE_unknown_1234, instead of just an empty string. It is based on
the formatv function, as that allows us to avoid materializing the
string for unknown values (and because this way I don't have to invent a
name for the new functions :P).

In this patch I add formatters for dwarf attributes, forms, tags, and
index attributes as these are the ones in use currently, but adding
other enums is straight-forward.

Reviewers: dblaikie, JDevlieghere, aprantl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44570

llvm-svn: 328090