Coverage: remove empty mapping regions

[arphaman]

Summary

This patch removes the empty coverage mapping regions.

Those regions were produced by clang's old mapping region generation algorithm, but the new algorithm doesn't generate them.

[bob.wilson]

Hard to argue with that one! Please go ahead and commit.

[arphaman]

Commited in r213981.

include/llvm/ProfileData/CoverageMapping.h

     /// such as for a macro instantiation or #include file.
     ExpansionRegion,
 
-    /// \brief An EmptyRegion represents a source range without code,
-    /// but with a distinct counter.
-    EmptyRegion,
-
     /// \brief A SkippedRegion represents a source range with code that
     /// was skipped by a preprocessor or similar means.
     SkippedRegion

lib/ProfileData/CoverageMappingReader.cpp

         case CounterMappingRegion::CodeRegion:
           // Don't do anything when we have a code region with a zero counter.
           break;
-        case CounterMappingRegion::EmptyRegion:
-          Kind = CounterMappingRegion::EmptyRegion;
-          break;
         case CounterMappingRegion::SkippedRegion:
           Kind = CounterMappingRegion::SkippedRegion;
           break;

lib/ProfileData/CoverageMappingWriter.cpp

       encodeULEB128(EncodedTagExpandedFileID, OS);
       break;
     }
-    case CounterMappingRegion::EmptyRegion:
-      encodeULEB128(unsigned(I.Kind)
-                        << Counter::EncodingCounterTagAndExpansionRegionTagBits,
-                    OS);
-      break;
     case CounterMappingRegion::SkippedRegion:
       assert(Count.isZero());
       encodeULEB128(unsigned(I.Kind)