Basic: Turn off solver expression time threshold by default

The check is expensive, the default of 10 minutes is absurd, and the
existing scope/trail/memory limits should now be sufficient.

Author: slavapestov

include/swift/Basic/LangOptions.h

@@ -852,7 +852,7 @@ namespace swift {
 
     /// If non-zero, abort the expression type checker if it takes more
     /// than this many seconds.
-    unsigned ExpressionTimeoutThreshold = 600;
+    unsigned ExpressionTimeoutThreshold = 0;
 
     /// The upper bound, in bytes, of temporary data that can be
     /// allocated by the constraint solver.