When we lower the instruction for `@tagName` we generate a new
function if it doesn't exist yet for that decl. This function creates
an if-else chain to determine which value was provided. Each tag
generates a constant that contains its name as the value. For each
tag we generate a case where the pointer of the string is stored in
the result slice. The length of the tagname is comptime-known, therefore
will be stored in the slice directly without having it being part of
the tagname symbol. In the future this can use a jump table instead
of an if-else chain, similar to the `switch` instruction.
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