Fix some cases where StringRef was being passed by const reference. Remove const from some other StringRefs since its implicitly const already.

llvm-svn: 216825
diff --git a/clang/lib/Driver/Compilation.cpp b/clang/lib/Driver/Compilation.cpp
index 0697285..a14b889 100644
--- a/clang/lib/Driver/Compilation.cpp
+++ b/clang/lib/Driver/Compilation.cpp
@@ -233,8 +233,8 @@
   // Redirect stdout/stderr to /dev/null.
   Redirects = new const StringRef*[3]();
   Redirects[0] = nullptr;
-  Redirects[1] = new const StringRef();
-  Redirects[2] = new const StringRef();
+  Redirects[1] = new StringRef();
+  Redirects[2] = new StringRef();
 }
 
 StringRef Compilation::getSysRoot() const {