[ConstraintSystem] Augment ExpressionTimer to carry the threshold

Instead of asking callers of `isExpired` to provide the threshold,
let's ask for that upfront. This change also allows us to check how
much time remains in the timer and build timers with different
thresholds without having to safe that information somewhere else.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -171,11 +171,18 @@ class ExpressionTimer {
   ASTContext &Context;
   llvm::TimeRecord StartTime;
 
+  /// The number of milliseconds from creation until
+  /// this timer is considered expired.
+  unsigned ThresholdInMillis;
+
   bool PrintDebugTiming;
   bool PrintWarning;
 
 public:
+  /// This constructor sets a default threshold defined for all expressions
+  /// via compiler flag `solver-expression-time-threshold`.
   ExpressionTimer(Expr *E, ConstraintSystem &CS);
+  ExpressionTimer(Expr *E, ConstraintSystem &CS, unsigned thresholdInMillis);
 
   ~ExpressionTimer();
 
@@ -196,9 +203,9 @@ class ExpressionTimer {
   // than the warning threshold.
   void disableWarning() { PrintWarning = false; }
 
-  bool isExpired(unsigned thresholdInMillis) const {
+  bool isExpired() const {
     auto elapsed = getElapsedProcessTimeInFractionalSeconds();
-    return unsigned(elapsed) > thresholdInMillis;
+    return unsigned(elapsed) > ThresholdInMillis;
   }
 };
 
@@ -5226,9 +5233,7 @@ class ConstraintSystem {
       return isExpressionAlreadyTooComplex= true;
     }
 
-    const auto timeoutThresholdInMillis =
-        getASTContext().TypeCheckerOpts.ExpressionTimeoutThreshold;
-    if (Timer && Timer->isExpired(timeoutThresholdInMillis)) {
+    if (Timer && Timer->isExpired()) {
       // Disable warnings about expressions that go over the warning
       // threshold since we're arbitrarily ending evaluation and
       // emitting an error.

lib/Sema/ConstraintSystem.cpp

@@ -44,12 +44,17 @@ using namespace inference;
 #define DEBUG_TYPE "ConstraintSystem"
 
 ExpressionTimer::ExpressionTimer(Expr *E, ConstraintSystem &CS)
-  : E(E),
-      Context(CS.getASTContext()),
+    : ExpressionTimer(
+          E, CS,
+          CS.getASTContext().TypeCheckerOpts.ExpressionTimeoutThreshold) {}
+
+ExpressionTimer::ExpressionTimer(Expr *E, ConstraintSystem &CS,
+                                 unsigned thresholdInMillis)
+    : E(E), Context(CS.getASTContext()),
       StartTime(llvm::TimeRecord::getCurrentTime()),
+      ThresholdInMillis(thresholdInMillis),
       PrintDebugTiming(CS.getASTContext().TypeCheckerOpts.DebugTimeExpressions),
-      PrintWarning(true) {
-}
+      PrintWarning(true) {}
 
 ExpressionTimer::~ExpressionTimer() {
   auto elapsed = getElapsedProcessTimeInFractionalSeconds();