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.
...a situation we get into with indexing. The way Xcode generates
indexing invocations is to take a build command and add additional
flags to it; in order for the Driver to produce a single frontend
command from /that/, it currently plans as if it's going to do a
whole-module -typecheck and then turns around and uses -primary-file
anyway. This is questionable practice, to be sure...
...but meanwhile, let's not crash by trying to access declarations
that haven't been type-checked yet.
rdar://problem/53117124
