[ARM, Asm] Add diagnostics for general-purpose register operands

This adds diagnostic strings for the ARM general-purpose register
classes, which will be used when these classes are expected by the
assembler, but the provided operand is not valid.

One of these, rGPR, requires C++ code to select the correct error
message, as that class contains different registers in pre-v8 and v8
targets. The rest can all have their diagnostic strings stored in the
tablegen description of them.

Differential revision: https://reviews.llvm.org/D36692

llvm-svn: 315303