Sema: Fix -warn-long-expression-type-checking when expression timer is turned off

My change 983b75e1cf broke
-warn-long-expression-type-checking because now the
ExpressionTimer is not instantiated by default and that
entire code path is skipped.

Change it so that if -warn-long-expression-type-checking
is passed in, we still start the timer, we just don't
ever consider it to have 'expired'.

Fixes rdar://problem/152998878.

include/swift/Sema/ConstraintSystem.h

@@ -249,6 +249,8 @@ private:
   bool PrintWarning;
 
 public:
+  const static unsigned NoLimit = (unsigned) -1;
+
   ExpressionTimer(AnchorType Anchor, ConstraintSystem &CS,
                   unsigned thresholdInSecs);
 
@@ -274,6 +276,9 @@ public:
   /// Return the remaining process time in seconds until the
   /// threshold specified during construction is reached.
   unsigned getRemainingProcessTimeInSeconds() const {
+    if (ThresholdInSecs == NoLimit)
+      return NoLimit;
+
     auto elapsed = unsigned(getElapsedProcessTimeInFractionalSeconds());
     return elapsed >= ThresholdInSecs ? 0 : ThresholdInSecs - elapsed;
   }