Fix use-of-temporary with StringRef in code coverage

[jordan_rose]

Summary

The fixed code is basically identical to the same loop below, which might indicate an opportunity for refactoring. I just wanted to fix the use-of-temporary issue.

Caught by adding a similar check to StringRef as r283798 did for ArrayRef. I'll be upstreaming that soon.

[vsk]

Thanks, lgtm. I've made a note to clean this up.

[jordan_rose]

Committed as r286122.

lib/CodeGen/CoverageMappingGen.cpp

     // Dump the coverage mapping data for this function by decoding the
     // encoded data. This allows us to dump the mapping regions which were
     // also processed by the CoverageMappingWriter which performs
     // additional minimization operations such as reducing the number of
     // expressions.
     std::vector<StringRef> Filenames;
     std::vector<CounterExpression> Expressions;
     std::vector<CounterMappingRegion> Regions;
+    llvm::SmallVector<std::string, 16> FilenameStrs;
     llvm::SmallVector<StringRef, 16> FilenameRefs;
+    FilenameStrs.resize(FileEntries.size());
     FilenameRefs.resize(FileEntries.size());
-    for (const auto &Entry : FileEntries)
-      FilenameRefs[Entry.second] = normalizeFilename(Entry.first->getName());
+    for (const auto &Entry : FileEntries) {
+      auto I = Entry.second;
+      FilenameStrs[I] = normalizeFilename(Entry.first->getName());
+      FilenameRefs[I] = FilenameStrs[I];
+    }
     RawCoverageMappingReader Reader(CoverageMapping, FilenameRefs, Filenames,
                                     Expressions, Regions);
     if (Reader.read())
       return;
     dump(llvm::outs(), NameValue, Expressions, Regions);
   }
 }