[Coverage] Make sorting criteria for CounterMappingRegions local.

[ikudrin]

Summary

Move the comparison function into the only place there it is used,
i.e. the call to std::stable_sort in CoverageMappingWriter::write().

Add sorting by region kinds as it is required to ensure stable order
in our tests and to simplify D23987.

[vsk]

From D23987:

I don't want to make more changes in the program's behaviour than needed.

Makes sense, this lgtm.

include/llvm/ProfileData/Coverage/CoverageMapping.h

struct CounterMappingRegion {
   inline std::pair<unsigned, unsigned> startLoc() const {
     return std::pair<unsigned, unsigned>(LineStart, ColumnStart);
   }
 
   inline std::pair<unsigned, unsigned> endLoc() const {
     return std::pair<unsigned, unsigned>(LineEnd, ColumnEnd);
   }
 
-  bool operator<(const CounterMappingRegion &Other) const {
-    if (FileID != Other.FileID)
-      return FileID < Other.FileID;
-    return startLoc() < Other.startLoc();
-  }
-
   bool contains(const CounterMappingRegion &Other) const {
     if (FileID != Other.FileID)
       return false;
     if (startLoc() > Other.startLoc())
       return false;
     if (endLoc() < Other.endLoc())
       return false;
     return true;

lib/ProfileData/Coverage/CoverageMappingWriter.cpp

 
 static void writeCounter(ArrayRef<CounterExpression> Expressions, Counter C,
                          raw_ostream &OS) {
   encodeULEB128(encodeCounter(Expressions, C), OS);
 }
 
 void CoverageMappingWriter::write(raw_ostream &OS) {
   // Sort the regions in an ascending order by the file id and the starting
-  // location.
-  std::stable_sort(MappingRegions.begin(), MappingRegions.end());
+  // location. Sort by region kinds to ensure stable order for tests.
+  std::stable_sort(
+      MappingRegions.begin(), MappingRegions.end(),
+      [](const CounterMappingRegion &LHS, const CounterMappingRegion &RHS) {
+        if (LHS.FileID != RHS.FileID)
+          return LHS.FileID < RHS.FileID;
+        if (LHS.startLoc() != RHS.startLoc())
+          return LHS.startLoc() < RHS.startLoc();
+        return LHS.Kind < RHS.Kind;
+      });
 
   // Write out the fileid -> filename mapping.
   encodeULEB128(VirtualFileMapping.size(), OS);
   for (const auto &FileID : VirtualFileMapping)
     encodeULEB128(FileID, OS);
 
   // Write out the expressions.
   CounterExpressionsMinimizer Minimizer(Expressions, MappingRegions);