Move string copyBuff from implmentation to string base class

[Joshua-Anderson]

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Change Description

Convert string copyBuff from a virtual function that was implemented by string implementations to a method in the base string class.

Rationale

Removes code duplication of the copyBuff function and reduces the chance of introducing an error when implementing a string class. It also was triggering clang-tidy warnings because calling a virtual function in a constructor is highly discouraged due to the chance of calling an unintended implementation of the virtual function.

[LeStarch]

@timcanham could you look at this, and speak to why this wasn't done previously?

[timcanham]

This is a good change. I think we didn't really notice it because it was mostly hidden in code generation. This will save a bunch of code...