[Driver] Make sure the deployment target is earlier than iOS 11 when
it is inferred from -isysroot.

This fixes a change that was inadvertently introduced in r309607.

rdar://problem/32230613

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309633 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit d00f966d136fcf99c3a38f4b17ae8bc97c75f543)

Conflicts:
lib/Driver/ToolChains/Darwin.cpp
2 files changed