[APInt] Enable APInt to support zero bit integers.

Motivation: APInt not supporting zero bit values leads to
a lot of special cases in various bits of code, particularly
when using APInt as a bit vector (where you want to start with
zero bits and then concat on more.  This is particularly
challenging in the CIRCT project, where the absence of zero-bit
ConstantOp forces duplication of ops and makes instcombine-like
logic far more complicated.

Approach: zero bit integers are weird.  There are two reasonable
approaches: either make it illegal to do general arithmetic on
them (e.g. sign extends), or treat them as as implicitly having
a zero value.  This patch takes the conservative approach, which
enables their use in bitvector applications.

Differential Revision: https://reviews.llvm.org/D109555