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;
   }

lib/Sema/ConstraintSystem.cpp

@@ -141,9 +141,18 @@ ConstraintSystem::~ConstraintSystem() {
 void ConstraintSystem::startExpressionTimer(ExpressionTimer::AnchorType anchor) {
   ASSERT(!Timer);
 
-  unsigned timeout = getASTContext().TypeCheckerOpts.ExpressionTimeoutThreshold;
+  const auto &opts = getASTContext().TypeCheckerOpts;
-  if (timeout == 0)
+  unsigned timeout = opts.ExpressionTimeoutThreshold;
-    return;
+
+  // If either the timeout is set, or we're asked to emit warnings,
+  // start the timer. Otherwise, don't start the timer, it's needless
+  // overhead.
+  if (timeout == 0) {
+    if (opts.WarnLongExpressionTypeChecking == 0)
+      return;
+
+    timeout = ExpressionTimer::NoLimit;
+  }
 
   Timer.emplace(anchor, *this, timeout);
 }

test/Constraints/warn_long_compile.swift

@@ -1,5 +1,4 @@
-// RUN: %target-typecheck-verify-swift -warn-long-expression-type-checking=1 -warn-long-function-bodies=1
+// RUN: %target-typecheck-verify-swift -warn-long-expression-type-checking=1 -solver-disable-shrink -warn-long-function-bodies=1
-// REQUIRES: rdar44305428
 @_silgen_name("generic_foo")
 func foo<T>(_ x: T) -> T
 
@@ -8,6 +7,6 @@ func foo(_ x: Int) -> Int
 
 func test(m: Double) -> Int {
   // expected-warning@-1 {{global function 'test(m:)' took}}
-  return Int(foo(Float(m) / 20) * 20 - 2) + 10
+  return ~(~(~(~(~(~(~(~(~(~(~(~(0))))))))))))
   // expected-warning@-1 {{expression took}}
 }