Make PDBSymbol's IPDBSymbol reference const.

llvm-svn: 228553
diff --git a/llvm/lib/DebugInfo/PDB/PDBSymbolPublicSymbol.cpp b/llvm/lib/DebugInfo/PDB/PDBSymbolPublicSymbol.cpp
index b56abb0..7a63051 100644
--- a/llvm/lib/DebugInfo/PDB/PDBSymbolPublicSymbol.cpp
+++ b/llvm/lib/DebugInfo/PDB/PDBSymbolPublicSymbol.cpp
@@ -15,7 +15,7 @@
 using namespace llvm;
 
 PDBSymbolPublicSymbol::PDBSymbolPublicSymbol(
-    IPDBSession &PDBSession, std::unique_ptr<IPDBRawSymbol> Symbol)
+    const IPDBSession &PDBSession, std::unique_ptr<IPDBRawSymbol> Symbol)
     : PDBSymbol(Session, std::move(Symbol)) {}
 
 void PDBSymbolPublicSymbol::dump(llvm::raw_ostream &OS) const {}