Remove top-level Clang -fsanitize= flags for optional ASan features.

Init-order and use-after-return modes can currently be enabled
by runtime flags. use-after-scope mode is not really working at the
moment.

The only problem I see is that users won't be able to disable extra
instrumentation for init-order and use-after-scope by a top-level Clang flag.
But this instrumentation was implicitly enabled for quite a while and
we didn't hear from users hurt by it.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210924 91177308-0d34-0410-b5e6-96231b3b80d8