[ARM] Use new assembler diags for ARM

This converts the ARM AsmParser to use the new assembly matcher error
reporting mechanism, which allows errors to be reported for multiple
instruction encodings when it is ambiguous which one the user intended
to use.

By itself this doesn't improve many error messages, because we don't have
diagnostic text for most operand types, but as we add that then this will allow
more of those diagnostic strings to be used when they are relevant.

Differential revision: https://reviews.llvm.org/D31530

llvm-svn: 314779