[MIR] Print target-specific constant pools

This should enable us to test the generation of target-specific constant
pools, e.g. for ARM:

constants:
 - id:              0
   value:           'g(GOT_PREL)-(LPC0+8-.)'
   alignment:       4
   isTargetSpecific: true

I intend to use this to test PIC support in GlobalISel for ARM.

This is difficult to test outside of that context, since the existing
MIR tests usually rely on parser support as well, and that seems a bit
trickier to add. We could try to add a unit test, but the setup for that
seems rather convoluted and overkill.

We do test however that the parser reports a nice error when
encountering a target-specific constant pool.

Differential Revision: https://reviews.llvm.org/D36092

llvm-svn: 309806