Refactor FunctionAttr to hold the internal function reference by name...

    Refactor FunctionAttr to hold the internal function reference by name instead of pointer. The one downside to this is that the function reference held by a FunctionAttr needs to be explicitly looked up from the parent module. This provides several benefits though:
    * There is no longer a need to explicitly remap function attrs.
      - This removes a potentially expensive call from the destructor of Function.
      - This will enable some interprocedural transformations to now run intraprocedurally.
      - This wasn't scalable and forces dialect defined attributes to override
        a virtual function.
    * Replacing a function is now a trivial operation.
    * This is a necessary first step to representing functions as operations.

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PiperOrigin-RevId: 249510802