Introduce "DefinedOrUnknownSVal" into the SVal class hierarchy, providing a way

to statically type various methods in SValuator/GRState as required either a
defined value or a defined-but-possibly-unknown value. This leads to various
logic cleanups in GRExprEngine, and lets the compiler enforce via type checking
our assumptions about what symbolic values are possibly undefined and what are
not.

Along the way, clean up some of the static analyzer diagnostics regarding the uses of uninitialized values.

llvm-svn: 81579