Remove leftover const.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166996 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Frontend/ASTUnit.h

@@ -467,7 +467,7 @@ public:
 
   const FileSystemOptions &getFileSystemOpts() const { return FileSystemOpts; }
 
-  const StringRef getOriginalSourceFileName() {
+  StringRef getOriginalSourceFileName() {
     return OriginalSourceFile;
   }