DWARFDebugLine.cpp: Format unknown line number standard opcodes

Summary:
This patch implements `formatv()` formatting for `dwarf::LineNumberOps`
and makes use of it for the `llvm-dwarfdump --debug-line` dump.

Previously, unknown line number standard opcodes would lead to undefined
behaviour. The code would attempt to format the data pointer of an empty
`StringRef` (a null pointer) using `%s`. According to the description
for `format()`, use of that interface carries the "risk of `printf`".
Passing a null pointer in place of an array to a C library function
results in undefined behaviour.

Reviewers: jhenderson, daltenty, stevewan

Reviewed By: jhenderson

Subscribers: aprantl, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72369