Improve llvm-mc disassembler mode and refactor ARM tests to use it

This allows "llvm-mc -disassemble" to accept two new features:
  + Using comma as a byte separator
  + Grouping bytes with '[' and ']' pairs.

The behaviour outside a [...] group is unchanged. But within the group once
llvm-mc encounters a true error, it stops rather than trying to resynchronise
the stream at the next byte. This is more useful for disassembly tests, where
we have an almost-instruction in mind and don't care what the misaligned
interpretation would be. Particularly if it means llvm-mc won't actually see
the next intended almost-instruction.

As a side effect, this means llvm-mc can disassemble its own -show-encoding
output if copy-pasted.