Default to use new swift-driver to run lit tests as legacy swift driver
is deprecated for a long time. The lit tests for legacy drivers are also
disable when running new swift-driver.
Tests that relies on legacy swift driver (for example, tests merge
module) can be enabled on conditions `legacy_swift_driver`. Fix and
split up some existing tests that relies on legacy driver.
A couple of tests are expecting merge-modules behaviour,
add `-no-emit-module-separately` in those cases to
ensure that merge modules is used in both the old
and new driver.
Steven Wu
Becca Royal-Gordon
Hamish Knight
Harlan Haskins