[analyzer] Avoid ProgramStateRef copy constructors.

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169563 91177308-0d34-0410-b5e6-96231b3b80d8