Backends should not need two passes to handle wrapped functions

[jimblandy]

Once #7012 lands, Naga's Metal Shading Language, HLSL, and SPIR-V backends will make a prepass over each function's expressions to see which wrapped functions it will need, and then generate their definitions before the function's own translation. This means that both the prepass and the actual code generation need to repeat the logic for deciding which expressions need wrapped functions.

Instead, we should generate output for the Module's functions, and note which wrapped functions we needed by adding them to the backend's wrapped_functions table. Then, after all the Module's functions have been translated, we should simply iterate over the wrapped_functions table and generate the definitions we need.

Since MSL and HLSL require the wrapped function definitions to precede their uses, in those backends the two stages should write to separate Strings, which the backend should concatenate as a final step. Those copies should be pretty cheap. SPIR-V allows OpFunctionCall to refer to functions defined later, so in that case no reordering is necessary.

[cwfitzgerald]

Should hlsl also do this?

[jimblandy]

Yes, and in SPIR-V as well. In that case it's trivial, because SPIR-V allows function calls to be forward references.

Edited first comment.