llvm-objdump: factor code better, add comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140153 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/llvm-objdump/MCFunction.h b/tools/llvm-objdump/MCFunction.h
index 4677d91..6d3a548 100644
--- a/tools/llvm-objdump/MCFunction.h
+++ b/tools/llvm-objdump/MCFunction.h
@@ -83,9 +83,9 @@
                        const MCInstrAnalysis *Ana, raw_ostream &DebugOut,
                        SmallVectorImpl<uint64_t> &Calls);
 
-  typedef MapTy::iterator iterator;
-  iterator begin() { return Blocks.begin(); }
-  iterator end() { return Blocks.end(); }
+  typedef MapTy::const_iterator iterator;
+  iterator begin() const { return Blocks.begin(); }
+  iterator end() const { return Blocks.end(); }
 
   StringRef getName() const { return Name; }