Fix an assertion failure when building for the iOS simulator. rdar://15959009

When building for i386 or x86_64 with IPHONEOS_DEPLOYMENT_TARGET set in the
environment, the toolchain correctly recognizes that the target platform is
the iOS simulator. The code in Darwin::addMinVersionArgs was not updated for
svn 197148, where isTargetIPhoneOS() was widely replaced by isTargetIOSBased().
This is kind of a strange case, though, because we probably ought to be
passing -ios_simulator_version_min to the linker, but according to the FIXME
in the code, we intentionally avoid that unless the -mios-simulator-version-min
option was used. I don't know whether it is safe to change that yet, so
for now, I am just fixing the assertion failure.