Fixup -ftrapv to be more gcc compatible. -ftrapu (for want of a

Fixup -ftrapv to be more gcc compatible.  -ftrapu (for want of a
better name) is the option that SmallTalk can use to intercept all
overflows, including unsigned.  I added some testcases so we don't
break anything.

Also included is another patch from David for += and friends.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@68267 91177308-0d34-0410-b5e6-96231b3b80d8