Accept Input Source Maps

The typescript compiler produces sources maps mapping from the javascript it produces to the typescript sources it compiled. This is great, but doesn't go far enough if there are tools which generate the sources that are consumed by the typescript compiler.

For example, Tsickle preprocesses typescript sources before passing them to the typescript compiler. As things stand we've had to implement a hack to get our source maps to work. We use a compiler host which performs the tsickle transforms as the typescript compiler reads files, saves the resulting source maps and then compose them with the source maps produced by the typescript compiler as they're emitted.

Instead the typescript compiler could accept source maps as input alongside sources, and compose the input source maps with the source maps produced by the compiler.

The ideal api would be that the typescript compiler would look for either a. an inline source map as a comment in the source file or b. a file in the same directory with the name {source file name}.ts.map, which shouldn't require any crazy file resolution.

The issue names no repository files or tests. Start by tracing the compiler host's source-map emission path and how transformed sources are read, then determine where inline maps or adjacent .ts.map files would be discovered and composed. Done means the compiler accepts both input-map forms and combines them with its emitted source maps.