Add a build tag to avoid disabling dead code elimination

[pgimalac]

Fixes #373.

This PR moves creating the string representation of an unnamed interface behind a build tag interface_string_no_reflect.

• When the build tag is not used, nothing changes.
• When the build tag is used, just use reflect.Type.String() to get the string representation of the type. The main caveat of this implementation is that it ignores the qualified argument.

Additional notes
This works because the compiler can determine that one branch of the condition is not reachable, due to using a constant and not a variable.

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