[mlir] build fix for gcc-5

[stephenneuendorffer]

Summary

It appears in this case that an implicit cast from StringRef to std::string
doesn't happen. Fixed with an explicit cast.

mlir/lib/IR/Dialect.cpp

Dialect *DialectRegistry::loadByName(StringRef name, MLIRContext *context) {
   if (it == registry.end())
     return nullptr;
   return it->second.second(context);
 }
 
 void DialectRegistry::insert(TypeID typeID, StringRef name,
                              DialectAllocatorFunction ctor) {
   auto inserted =
-      registry.insert(std::make_pair(name, std::make_pair(typeID, ctor)));
+    registry.insert(std::make_pair((std::string)name,
+                                   std::make_pair(typeID, ctor)));
   if (!inserted.second && inserted.first->second.first != typeID) {
     llvm::report_fatal_error(
         "Trying to register different dialects for the same namespace: " +
         name);
   }
 }
 
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