Consider unteaching ServiceCodeGenerator about sources vs. headers

[dbolduc]

We could have one ServiceCodeGenerator per file, instead of one ServiceCodeGenerator per potential pairs of files.

ServiceCodeGenerator writes files. It takes care to distinguish between headers and sources. It has a separate Printer for source and header. It has many CcFoo() / HeaderFoo() member functions. But the implementation of these member functions are basically the same.

Why we would do this:

• I am about to introduce a hacky overload because I want a ServiceCodeGenerator that only writes a source file.
• Simplify the class. I suspect it will be easier to navigate. (I often forget whether I am in GenerateHeader() or GenerateCc())
• We could parallelize writing headers and sources. The savings, if any, would be negligible.

Why we wouldn't do this:
If there was a ton of common code shared between headers and sources, then having one class to control both makes sense.

• The client generators have a shared initialization for {get,set}_iam_policy_extension_.
• The connection generators share a ConnectionFactoryFunctionArguments()

We could always factor this stuff out. The processing could happen in make_generators.cc, instead of inside the generator. e.g. Each ServiceCodeGenerator calls SetMethods() which does the same work to produce its methods_, async_methods_. That could also happen once outside of the ServiceCodeGenerators.