Annotate APInt methods where it's not clear whether they are in place with warn_unused_result.

Fix ScalarEvolution bugs uncovered by this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194928 91177308-0d34-0410-b5e6-96231b3b80d8
ScalarEvolution: Warn if the result of setFlags/clearFlags is unused.

This was a source of bugs in the past.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194929 91177308-0d34-0410-b5e6-96231b3b80d8