[analyzer] Use makeZeroVal in RegionStore's lazy evaluation of statics.

No functionality change.
[analyzer] Use Clang's evaluation for global constants and default arguments.

Previously, we were handling only simple integer constants for globals and
the smattering of implicitly-valued expressions handled by Environment for
default arguments. Now, we can use any integer constant expression that
Clang can evaluate, in addition to everything we handled before.

PR15094 / <rdar://problem/12830437>