[llvm-cov] Emit a summary in the report directory's index

llvm-cov writes out an index file in '-output-dir' mode, albeit not a
very informative one. Try to fix that by using the CoverageReport API to
include some basic summary information in the index file.

llvm-svn: 281011
diff --git a/llvm/tools/llvm-cov/SourceCoverageViewText.h b/llvm/tools/llvm-cov/SourceCoverageViewText.h
index fe2642a..6fd35f5 100644
--- a/llvm/tools/llvm-cov/SourceCoverageViewText.h
+++ b/llvm/tools/llvm-cov/SourceCoverageViewText.h
@@ -26,7 +26,8 @@
 
   void closeViewFile(OwnedStream OS) override;
 
-  Error createIndexFile(ArrayRef<StringRef> SourceFiles) override;
+  Error createIndexFile(ArrayRef<StringRef> SourceFiles,
+                        const coverage::CoverageMapping &Coverage) override;
 
   CoveragePrinterText(const CoverageViewOptions &Opts)
       : CoveragePrinter(Opts) {}