Fix compile on older clang/OSX versions

Apparently there's a bug in older clang versions which rejects this code, and
the fix is to just remove the override keyword for now. This is tracked in this
llvm bug:

  https://llvm.org/bugs/show_bug.cgi?id=22619

Author: alexcrichton

include/llvm/ExecutionEngine/Orc/LazyEmittingLayer.h

@@ -123,7 +123,7 @@ template <typename BaseLayerT> class LazyEmittingLayer {
   protected:
 
     const GlobalValue* searchGVs(StringRef Name,
-                                 bool ExportedSymbolsOnly) const override {
+                                 bool ExportedSymbolsOnly) const {
       // FIXME: We could clean all this up if we had a way to reliably demangle
       //        names: We could just demangle name and search, rather than
       //        mangling everything else.