Fix MSVC "illegal conversion; more than one user-defined conversion has been implicitly applied" warning. NFCI.

llvm-svn: 342125
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index e84c755..908ffa3 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -4163,7 +4163,7 @@
       SmallString<128> P(Dir[0] == '=' ? SysRoot + Dir.substr(1) : Dir);
       llvm::sys::path::append(P, Name);
       if (llvm::sys::fs::exists(Twine(P)))
-        return {P.str()};
+        return P.str().str();
     }
     return None;
   };