[mlir][AsmPrinter] Fix multi-threaded segfault by using explicit null stream per thread

[rriddle]

Summary

We were using llvm::nulls, but that isn't thread safe so we switch to giving each thread it's own null stream.

mlir/lib/IR/AsmPrinter.cpp

for (const NamedAttribute &attr : attrs)
         printAttribute(attr.second);
   }
   void printOptionalAttrDictWithKeyword(
       ArrayRef<NamedAttribute> attrs,
       ArrayRef<StringRef> elidedAttrs = {}) override {
     printOptionalAttrDict(attrs, elidedAttrs);
   }
 
-  /// Return 'nulls' as the output stream, this will ignore any data fed to it.
-  raw_ostream &getStream() const override { return llvm::nulls(); }
+  /// Return a null stream as the output stream, this will ignore any data fed
+  /// to it.
+  raw_ostream &getStream() const override { return os; }
 
   /// The following are hooks of `OpAsmPrinter` that are not necessary for
   /// determining potential aliases.
   void printAffineMapOfSSAIds(AffineMapAttr, ValueRange) override {}
   void printNewline() override {}
   void printOperand(Value) override {}
   void printOperand(Value, raw_ostream &os) override {
     // Users expect the output string to have at least the prefixed % to signal
     // a value name. To maintain this invariant, emit a name even if it is
     // guaranteed to go unused.
     os << "%";
   }
   void printSymbolName(StringRef) override {}
   void printSuccessor(Block *) override {}
   void printSuccessorAndUseList(Block *, ValueRange) override {}
   void shadowRegionArgs(Region &, ValueRange) override {}
 
   /// The printer flags to use when determining potential aliases.
   const OpPrintingFlags &printerFlags;
 
   /// The initializer to use when identifying aliases.
   AliasInitializer &initializer;
+
+  /// A dummy output stream.
+  mutable llvm::raw_null_ostream os;
 };
 } // end anonymous namespace
 
 /// Sanitize the given name such that it can be used as a valid identifier. If
 /// the string needs to be modified in any way, the provided buffer is used to
 /// store the new copy,
 static StringRef sanitizeIdentifier(StringRef name, SmallString<16> &buffer,
                                     StringRef allowedPunctChars = "$._-",