Type Promotion nodes should cache pins that have had their type manually converted by the user

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We should just cache any user-converted pins on the type promotion nodes and make sure to keep those types if they are valid. If they are invalid, then we can orphan them or something like that. 

1. Make a multiple node
2. Connect a vector to it
3. Convert the second input pin to a float
4. Connect the output pin

Result: The input pin is reset to a vector type, because the default value has not been changed.

Expected: The second input pin stays as a float type after the output is connected