[analyzer] Avoid ProgramStateRef copy constructors.

Suggested by David Blaikie. ExplodedNode, CallEvent, and CheckerContext all
hang onto their ProgramState, so the accessors can return a reference to the
internal state rather than preemptively copying it. This helps avoid
temporary ProgramStateRefs, though local variables will still (correctly)
do an extra retain and release.

llvm-svn: 169563