[llvm] r186661 - Improve llvm-mc disassembler mode and refactor ARM tests to use it

[Tim Northover]

> This causes llvm-mc to print a warning and exit with 1. Is that really
> intentional? (see r186939).

I think so. The warning is certainly intentional (and unless I
misunderstand explicitly emitted in the lines just above) because it's
what's actually being checked by the tests.

The error code I'm less sure of. The output *is* even less likely to
make sense than otherwise if that's hit, but on the other hand llvm-mc
has probably done exactly what was needed.

I'm not that bothered either way, really.

Cheers.

Tim.