Add diagnostics files to OutputFileMap, but grumble about it.

`OutputFileMap` is effectively growing a copy of `FileType` in it, which is
unfortunate. Add yet one more entry (for serialized diagnostics files), but
because this isn’t a robust solution, add a TODO to the README to generalize
the implementation.

Author: DougGregor

README.md

@@ -132,6 +132,7 @@ The goal of the new Swift driver is to provide a drop-in replacement for the exi
   * [x] Parseable output, as used by SwiftPM
   * [x] Response files
   * [ ] Input and primary input file lists
+  * [ ] Complete `OutputFileMap` implementation to handle all file types uniformly
 * Testing
   * [ ] Build stuff with SwiftPM or Xcode or your favorite build system, using `swift-driver`. Were the results identical? What changed?
   * [x] Shim in `swift-driver` so it can run the Swift repository's [driver test suite](https://github.com/apple/swift/tree/master/test/Driver).

Sources/SwiftDriver/Driver/OutputFileMap.swift

@@ -115,13 +115,15 @@ fileprivate struct OutputFileMapJSON: Codable {
       case swiftmodule
       case swiftinterface
       case swiftDependencies = "swift-dependencies"
+      case diagnostics
     }
 
     let dependencies: String?
     let object: String?
     let swiftmodule: String?
     let swiftinterface: String?
     let swiftDependencies: String?
+    let diagnostics: String?
   }
 
   /// The parsed entires
@@ -150,6 +152,7 @@ fileprivate struct OutputFileMapJSON: Codable {
       map[.swiftModule] = entry.swiftmodule
       map[.swiftInterface] = entry.swiftinterface
       map[.swiftDeps] = entry.swiftDependencies
+      map[.diagnostics] = entry.diagnostics
 
       result[input] = try map.mapValues(VirtualPath.init(path:))
     }
@@ -172,7 +175,8 @@ fileprivate struct OutputFileMapJSON: Codable {
         object: outputs[.object]?.name,
         swiftmodule: outputs[.swiftModule]?.name,
         swiftinterface: outputs[.swiftInterface]?.name,
-        swiftDependencies: outputs[.swiftDeps]?.name)
+        swiftDependencies: outputs[.swiftDeps]?.name,
+        diagnostics: outputs[.diagnostics]?.name)
     }
     return Self(entries: Dictionary(uniqueKeysWithValues: entries.map(convert(entry:))))
   }