[Coverage][Unittest] Fix stringref issue

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.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D97353

unittests/ProfileData/CoverageMappingTest.cpp

@@ -201,6 +201,9 @@ struct CoverageMappingTest : ::testing::TestWithParam<std::pair<bool, bool>> {
 
   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();