Add -f[no-]strict-overflow to the Clang driver. Use it to set the

default for -fwrapv if that flag isn't specified explicitly. We always
prefer an explict setting of -fwrapv when present. Also adds support for
-fno-wrapv to allow disabling -fwrapv even when -fno-strict-overflow is
passed.

llvm-svn: 128353