[analyzer] Annotate ProgramState update methods with LLVM_NODISCARD.

Because our program states are immutable, methods like "add<>", "set<>", "bind"
create a copy of the program state instead of mutating the existing state.
If the updated state is discarded, it clearly indicates a bug.
Such bugs are introduced frequently, hence the warn_unused_result annotation.

Differential Revision: https://reviews.llvm.org/D47499

llvm-svn: 333679