c-index-test sorts the units by their file name, which for the modules in
in this test is just [target-triple].swiftinterface-[hash-of-full-output-path]
and so changes depending on where it's run.
Several tests related to indexing system modules were taking a considerable
amount of time (100+ seconds in the worst case) indexing the standard library.
This adds a frontend option to skip it and updates those tests to pass it.
