Make use of DiagnosticType to provide better AArch64 diagnostics.

This gives a DiagnosticType to all AsmOperands in sight. This replaces all
"invalid operand" diagnostics with something more specific. The messages given
should still be sufficiently vague that they're not usually actively misleading
when LLVM guesses your instruction incorrectly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174871 91177308-0d34-0410-b5e6-96231b3b80d8