Don't re-instantiate the std::string in catch block.

llvm-svn: 16159
diff --git a/llvm/tools/llvmc/llvmc.cpp b/llvm/tools/llvmc/llvmc.cpp
index f457795..9d677d7 100644
--- a/llvm/tools/llvmc/llvmc.cpp
+++ b/llvm/tools/llvmc/llvmc.cpp
@@ -323,7 +323,7 @@
 
     // All is good, return success
     return 0;
-  } catch (std::string& msg) {
+  } catch (const std::string& msg) {
     std::cerr << argv[0] << ": " << msg << "\n";
   } catch (...) {
     std::cerr << argv[0] << ": Unexpected unknown exception occurred.\n";