[IR][CodeGen] Add new SizedCapabilityType for use in CodeGen

[jrtc27]

EVT::getTypeForEVT currently returns a PointerType for MVT::cN, but this
has a couple of issues. The first issue is we have to hard-code the
address space, though that's not such a big deal given we do that
elsewhere too. The bigger issue is that, when we later pass that to
MVT::getVT or EVT::getEVT, it doesn't know what the right size is, so
returns MVT::cPTR instead, which is not a true value type, and is
supposed to only be used by TableGen. This has been seen to confuse
TargetLoweringBase::getTypeConversion, as when presented with a vector
of capability pointers it can end up trying to recreate a smaller vector
of the same type, but this trips up various assertions for the MVT::cPTR
as both the IR methods and the code here are expecting to be dealing
with actual value types.

Borrowing the idea of TypedPointerType (DXILPointerTyID) a bit,
introduce a new IR type, SizedCapabilityType, to represent a fixed-size
capability during CodeGen, which allows lossless roundtripping from MVT
to Type and back.

This fixes building cheritest, which has crashed since the introduction
of cPTR due to cPTR not being a value type, mirroring iPTR, unlike the
old iFATPTRAny which was its own weird beast, but wouldn't have tripped
up these assertions. It probably didn't do the most sensible things
though.

Fixes: 7aa7f2e ("[CodeGen] Rework MVT representation of capabilities and add type inference")

[arichardson]

Sorry for the delay here, too many GitHub notifications while I was out of office so somehow missed this one.