[NFC][support]Add operator- to TimeRecord (llvm#163361)

A common use case for the Timer is to measure time duration between two
points. Lack of operator- forced an non-intuitive two-step duration
computation: get the end time, then decrement it by the start time.

Now, as demonstrated on the example of `SyntaxCheckTimer` and
`ExprEngineTimer` one can compute duration directly.

Author: necto

clang/lib/StaticAnalyzer/Frontend/AnalysisConsumer.cpp

@@ -760,11 +760,11 @@ void AnalysisConsumer::HandleCode(Decl *D, AnalysisMode Mode,
     ++NumFunctionsAnalyzedSyntaxOnly;
     if (SyntaxCheckTimer) {
       SyntaxCheckTimer->stopTimer();
-      llvm::TimeRecord CheckerEndTime = SyntaxCheckTimer->getTotalTime();
-      CheckerEndTime -= CheckerStartTime;
+      llvm::TimeRecord CheckerDuration =
+          SyntaxCheckTimer->getTotalTime() - CheckerStartTime;
       FunctionSummaries.findOrInsertSummary(D)->second.SyntaxRunningTime =
-          std::lround(CheckerEndTime.getWallTime() * 1000);
-      DisplayTime(CheckerEndTime);
+          std::lround(CheckerDuration.getWallTime() * 1000);
+      DisplayTime(CheckerDuration);
       if (AnalyzerTimers && ShouldClearTimersToPreventDisplayingThem) {
         AnalyzerTimers->clear();
       }
@@ -825,11 +825,11 @@ void AnalysisConsumer::RunPathSensitiveChecks(Decl *D,
                       Mgr->options.MaxNodesPerTopLevelFunction);
   if (ExprEngineTimer) {
     ExprEngineTimer->stopTimer();
-    llvm::TimeRecord ExprEngineEndTime = ExprEngineTimer->getTotalTime();
-    ExprEngineEndTime -= ExprEngineStartTime;
+    llvm::TimeRecord ExprEngineDuration =
+        ExprEngineTimer->getTotalTime() - ExprEngineStartTime;
     PathRunningTime.set(static_cast<unsigned>(
-        std::lround(ExprEngineEndTime.getWallTime() * 1000)));
-    DisplayTime(ExprEngineEndTime);
+        std::lround(ExprEngineDuration.getWallTime() * 1000)));
+    DisplayTime(ExprEngineDuration);
     if (AnalyzerTimers && ShouldClearTimersToPreventDisplayingThem) {
       AnalyzerTimers->clear();
     }

llvm/include/llvm/Support/Timer.h

@@ -66,6 +66,12 @@ class TimeRecord {
     MemUsed -= RHS.MemUsed;
     InstructionsExecuted -= RHS.InstructionsExecuted;
   }
+  TimeRecord operator-(const TimeRecord &RHS) const {
+    TimeRecord R = *this;
+    R -= RHS;
+    return R;
+  }
+  // Feel free to add operator+ if you need it
 
   /// Print the current time record to \p OS, with a breakdown showing
   /// contributions to the \p Total time record.