Constify the Function pointers in the result of makeSubprogramMap

These don't need to be mutable and callers being added soon in CodeGen
won't have access to non-const Module&.

llvm-svn: 212202
diff --git a/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp b/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
index 2e89a25..97a119b 100644
--- a/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
+++ b/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
@@ -84,7 +84,7 @@
     bool doInitialization(CallGraph &CG) override;
     /// The maximum number of elements to expand, or 0 for unlimited.
     unsigned maxElements;
-    DenseMap<Function *, DISubprogram> FunctionDIs;
+    DenseMap<const Function *, DISubprogram> FunctionDIs;
   };
 }