[Attributor] Deduce "nosync" function attribute

[sstefan1]

Summary

Introduce new "nosync" attribute which indicates that a function does not synchronize with other functions. Will be used with "nofree" (D49165 & D62687) for dereferencable.

[sstefan1]

changed updateImpl, the logic was wrong.

[sstefan1]

update

[sstefan1]

wrong diff

llvm/lib/Transforms/IPO/Attributor.cpp

struct AANoSyncFunction : AbstractAttribute, BooleanState {
   const AbstractState &getState() const override { return *this; }
   /// }
 
   /// See AbstractAttribute::getManifestPosition().
   virtual ManifestPosition getManifestPosition() const override {
     return MP_FUNCTION;
   }
 
-
   /// See AbstractAttribute::getAsStr().
   virtual const std::string getAsStr() const override {
     return getAssumed() ? "nosync" : "may-sync";
   }
 
   /// See AbstractAttribute::updateImpl(...).
   virtual ChangeStatus updateImpl(Attributor &A) override;