[APInt] Cast calls to add/sub/mul overflow methods to void if only their...

[APInt] Cast calls to add/sub/mul overflow methods to void if only their overflow bool out param is used.

This is preparation for a clang change to improve the [[nodiscard]] warning to not be ignored on methods that return a class marked [[nodiscard]] that are defined in the class itself. See D32207.

We should consider adding wrapper methods to APInt that return the overflow flag directly and discard the APInt result. This would eliminate the void casts and the need to create a bool before the call to pass to the out param.

llvm-svn: 300758