We already disabled one of these because it failed (extremely) rarely in
CI, but the assumption was that there was a bug. In reality, the
behaviour was correct, so remove the unreliable checks for good and
document why in the test.
rdar://36408114
... and add a few basic statistics about the number of requests, ASTs
built, etc. The Statistic type is loosely based on the one from LLVM,
but suitable for using without DEBUG macros and using SourceKit UIdents
to identify the statistic. The easiest way to add a new statistic is to
add it to SwiftStatistics.def in the SwiftLangSupport.
