llvm-cov: Rename a variable and clean up its usage

Offset is a terrible name for an indentation / nesting level, and it
confuses me every time I look at this code.

llvm-svn: 217861
diff --git a/llvm/tools/llvm-cov/SourceCoverageView.h b/llvm/tools/llvm-cov/SourceCoverageView.h
index 362ff1a..0777c23 100644
--- a/llvm/tools/llvm-cov/SourceCoverageView.h
+++ b/llvm/tools/llvm-cov/SourceCoverageView.h
@@ -141,7 +141,7 @@
   void renderLine(raw_ostream &OS, StringRef Line,
                   ArrayRef<HighlightRange> Ranges);
 
-  void renderOffset(raw_ostream &OS, unsigned I);
+  void renderIndent(raw_ostream &OS, unsigned Level);
 
   void renderViewDivider(unsigned Offset, unsigned Length, raw_ostream &OS);
 
@@ -192,7 +192,7 @@
 
   /// \brief Print the code coverage information for a specific
   /// portion of a source file to the output stream.
-  void render(raw_ostream &OS, unsigned Offset = 0);
+  void render(raw_ostream &OS, unsigned IndentLevel = 0);
 
   /// \brief Load the coverage information required for rendering
   /// from the mapping regions in the data manager.