[Timers] TimerGroup::printJSONValue(): print doubles with no precision loss

[lebedev.ri]

Summary

Although this is not stricly required, i would very much prefer
not to have known random precision losses along the way.

[george.karpenkov]

LGTM

llvm/trunk/lib/Support/Timer.cpp

 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/Mutex.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/YAMLTraits.h"
 #include "llvm/Support/raw_ostream.h"
+#include <limits>
+
 using namespace llvm;
 
 // This ugly hack is brought to you courtesy of constructor/destructor ordering
 // being unspecified by C++.  Basically the problem is that a Statistic object
 // gets destroyed, which ends up calling 'GetLibSupportInfoOutputFile()'
 // (below), which calls this function.  LibSupportInfoOutputFilename used to be
 // a global variable, but sometimes it would get destroyed before the Statistic,
 // causing havoc to ensue.  We "fix" this by creating the string the first time
void TimerGroup::printAll(raw_ostream &OS) {
 
   for (TimerGroup *TG = TimerGroupList; TG; TG = TG->Next)
     TG->print(OS);
 }
 
 void TimerGroup::printJSONValue(raw_ostream &OS, const PrintRecord &R,
                                 const char *suffix, double Value) {
   assert(yaml::needsQuotes(Name) == yaml::QuotingType::None &&
-         "TimerGroup name needs no quotes");
+         "TimerGroup name should not need quotes");
   assert(yaml::needsQuotes(R.Name) == yaml::QuotingType::None &&
-         "Timer name needs no quotes");
-  OS << "\t\"time." << Name << '.' << R.Name << suffix << "\": " << Value;
+         "Timer name should not need quotes");
+  constexpr auto max_digits10 = std::numeric_limits<double>::max_digits10;
+  OS << "\t\"time." << Name << '.' << R.Name << suffix
+     << "\": " << format("%.*e", max_digits10 - 1, Value);
 }
 
 const char *TimerGroup::printJSONValues(raw_ostream &OS, const char *delim) {
   prepareToPrintList();
   for (const PrintRecord &R : TimersToPrint) {
     OS << delim;
     delim = ",\n";