We've been collecting the location info for some time now, but
apparently never printed it in no-assert builds of the stdlib, which
means this functionality was never available to the users.
With this change, the location will be printed depending on the
debug/release build configuration of the program, not stdlib.
Somewhat addresses: <rdar://problem/42980523>
Modifies SILGen and the `Swift._diagnoseUnexpectedNilOptional` call to print a slightly different message for force unwraps which were implicitly inserted by the compiler for IUOs. The message is chosen based on the presence of certain flags in the `ForceValueExpr`, not on the type of the value being unwrapped.
This decreases total testing time by over a minute on my old Mac Pro.
It probably has much less effect on systems with fewer cores, but shouldn't
be any worse there.
Swift SVN r22745
