Support indexing of multiple files in a single swift-frontend invocation

[ahoppen]

We can probably make indexing quite a bit more efficient by allowing multiple files from the same target to be indexed in a single swift-frontend invocation. That way the ASTContext can be re-used between those files.

We need to figure out how we set the output file name for these index tasks. They are currently the indexed .swift file.

rdar://127474313