SwiftSourceInfo files provide source location information for decls coming from
loaded modules. For most IDE use cases it either has an undesirable impact on
performance with no benefit (code completion), results in stale locations being
used instead of more up-to-date indexer locations (cursor info), or has no
observable effect (live diagnostics, which are filtered to just those with a
location in the primary file).
For non-IDE clients of SourceKit though, cursor info providing declaration
locations for symbols from other modules is useful, so add a global
configuration option (and a new request to set it) to control whether
.swiftsourceinfo files are loaded or not based on use case (they are loaded by
default).
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.
