Enable Reify types to have comments associated with them.

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The Reify interface definition should include support for associating comments with the defined types. This way, code generators can access comment information and include that in the generated files so that the information is available in many different forms.

This would, for example, enable TypeScript and C++ code completion tools to automatically display help text when typing out a function call.

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This is done now in 0db5319 . Though, because we use the TypeScript compiler to generate the ".d.ts" files from the original ".ts" files that Reify generates, the comments unfortunately are not currently carried over into the ".d.ts" files.