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
