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