[Sanitizer] iOS: Pull up parallelism_group handling into common.lit.config

Serial execution on iOS devices is not specific to sanitizers. We want
to throttle all on-device tests. Pull the setting of the
parallelism_group up into the common lit configuration file.

Rename `darwin-ios-device-sanitizer` to `ios-device`. This group is not
specific to sanitizers and (theoretically) independent from the host OS.

Note that we don't support running unit tests on-device (there are no
configurations generated for that). If that ever changes, we also need
this configuration in `unittests/lit.common.unit.cfg`.

Reviewers: delcypher

Differential Revision: https://reviews.llvm.org/D58209

llvm-svn: 354179