[ARM, Asm] Add diagnostics for floating-point register operands

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

One of these, DPR, requires C++ code to select the correct error
message, as that class contains different registers depending on the
FPU. The rest can all have their diagnostic strings stored in the
tablegen decription of them.

Differential revision: https://reviews.llvm.org/D36693

llvm-svn: 315304