[Support] Add a public API to allow clearing all (static) timer groups.

[graydon]

Summary

Formerly, all timer groups were automatically cleared when printed out. In
https://reviews.llvm.org/rL324788 this behaviour was changed to not-clearing
timers on printout, to allow printing timers more than once, but as a result
clients (specifically Swift) that relied on the clear-on-print behaviour to
inhibit duplicate timer printing on shutdown were broken.

Rather than revert that change, this change adds a new API that enables
clients that _want_ to clear all timers to do so explicitly.

[george.karpenkov]

Thanks for fixing up the comments!

Ideally, the test in Support/TimerTest should be updated as well, but I understand that it does not have any tests on TimerGroup yet.

llvm/trunk/include/llvm/Support/Timer.h

public:
 
   ~TimerGroup();
 
   void setName(StringRef NewName, StringRef NewDescription) {
     Name.assign(NewName.begin(), NewName.end());
     Description.assign(NewDescription.begin(), NewDescription.end());
   }
 
-  /// Print any started timers in this group and zero them.
+  /// Print any started timers in this group.
   void print(raw_ostream &OS);
 
-  /// This static method prints all timers and clears them all out.
+  /// Clear all timers in this group.
+  void clear();
+
+  /// This static method prints all timers.
   static void printAll(raw_ostream &OS);
 
+  /// Clear out all timers. This is mostly used to disable automatic
+  /// printing on shutdown, when timers have already been printed explicitly
+  /// using \c printAll or \c printJSONValues.
+  static void clearAll();
+
   const char *printJSONValues(raw_ostream &OS, const char *delim);
 
-  /// Prints all timers as JSON key/value pairs, and clears them all out.
+  /// Prints all timers as JSON key/value pairs.
   static const char *printAllJSONValues(raw_ostream &OS, const char *delim);
 
   /// Ensure global timer group lists are initialized. This function is mostly
   /// used by the Statistic code to influence the construction and destruction
   /// order of the global timer lists.
   static void ConstructTimerLists();
 private:
   friend class Timer;

llvm/trunk/lib/Support/Timer.cpp

void TimerGroup::PrintQueuedTimers(raw_ostream &OS) {
   Total.print(Total, OS);
   OS << "Total\n\n";
   OS.flush();
 
   TimersToPrint.clear();
 }
 
 void TimerGroup::prepareToPrintList() {
-  // See if any of our timers were started, if so add them to TimersToPrint and
-  // reset them.
+  // See if any of our timers were started, if so add them to TimersToPrint.
   for (Timer *T = FirstTimer; T; T = T->Next) {
     if (!T->hasTriggered()) continue;
     bool WasRunning = T->isRunning();
     if (WasRunning)
       T->stopTimer();
 
     TimersToPrint.emplace_back(T->Time, T->Name, T->Description);
 
     if (WasRunning)
       T->startTimer();
   }
 }
 
 void TimerGroup::print(raw_ostream &OS) {
   sys::SmartScopedLock<true> L(*TimerLock);
 
   prepareToPrintList();
 
   // If any timers were started, print the group.
   if (!TimersToPrint.empty())
     PrintQueuedTimers(OS);
 }
 
+void TimerGroup::clear() {
+  sys::SmartScopedLock<true> L(*TimerLock);
+  for (Timer *T = FirstTimer; T; T = T->Next)
+    T->clear();
+}
+
 void TimerGroup::printAll(raw_ostream &OS) {
   sys::SmartScopedLock<true> L(*TimerLock);
 
   for (TimerGroup *TG = TimerGroupList; TG; TG = TG->Next)
     TG->print(OS);
 }
 
+void TimerGroup::clearAll() {
+  sys::SmartScopedLock<true> L(*TimerLock);
+  for (TimerGroup *TG = TimerGroupList; TG; TG = TG->Next)
+    TG->clear();
+}
+
 void TimerGroup::printJSONValue(raw_ostream &OS, const PrintRecord &R,
                                 const char *suffix, double Value) {
   assert(yaml::needsQuotes(Name) == yaml::QuotingType::None &&
          "TimerGroup name should not need quotes");
   assert(yaml::needsQuotes(R.Name) == yaml::QuotingType::None &&
          "Timer name should not need quotes");
   constexpr auto max_digits10 = std::numeric_limits<double>::max_digits10;
   OS << "\t\"time." << Name << '.' << R.Name << suffix