When the compiler in `compile_commands.json` references a `swift` executable that’s a symlink to `swiftly`, SourceKit-LSP got confused because the `swift` executable doesn’t reside in a real toolchain, causing us to not provide any semantic functionality.
When we discover that the `swift` executable reference in compile commands references a `swiftly` executable, use `swiftly use -p` to resolve the binary in the real toolchain and continue operating based on that.
Fixes#2128
rdar://150301344
We need a mapping from source file to its output path in order to support source files that are part of multiple targets (because we need the output path to check if we have an up-to-date unit for a file in a given target). To achieve this mapping, it’s easier to tag the output path for each source file onto the `buildTarget/sources` request.
I feel like the implementations are actually simpler if we split them. This will also allow us to add more advanced logic to the JSON compilation database build system in the future, such as inferring the toolchain from the compile command.
