[Coverage][Unittest] Fix stringref issue

[jsji]

Summary

We will pass StringRef and change it in reader.
But we reuse the same Filename vector without clear it,
so in some systems, we may clobbeer previous results.

[phosek]

LGTM, thanks a lot for looking into this!

llvm/unittests/ProfileData/CoverageMappingTest.cpp

204–205

Would it be possible to move the comment on the line above?

[jsji]

Update comments.

llvm/unittests/ProfileData/CoverageMappingTest.cpp

std::string writeCoverageRegions(InputFunctionCoverageData &Data) {
     std::string Coverage;
     llvm::raw_string_ostream OS(Coverage);
     CoverageMappingWriter(FileIDs, None, Data.Regions).write(OS);
     return OS.str();
   }
 
   void readCoverageRegions(const std::string &Coverage,
                            OutputFunctionCoverageData &Data) {
+    // We will re-use the StringRef in duplicate tests, clear it to avoid
+    // clobber previous ones.
+    Filenames.clear();
     Filenames.resize(Files.size() + 1);
     for (const auto &E : Files)
       Filenames[E.getValue()] = E.getKey().str();
     std::vector<CounterExpression> Expressions;
     ArrayRef<std::string> FilenameRefs = llvm::makeArrayRef(Filenames);
     RawCoverageMappingReader Reader(Coverage, FilenameRefs, Data.Filenames,
                                     Expressions, Data.Regions);
     EXPECT_THAT_ERROR(Reader.read(), Succeeded());