ARM: Add a better diagnostic for some out of range immediates.

ARM: Add a better diagnostic for some out of range immediates.

As an example of how the custom DiagnosticType can be used to provide
better operand-mismatch diagnostics, add a custom diagnostic for
the imm0_15 operand class used for several system instructions.
Update the tests to expect the improved diagnostic.

rdar://8987109

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159051 91177308-0d34-0410-b5e6-96231b3b80d8