Implement a CLI to demangle symbols

Package.swift

@@ -51,6 +51,13 @@ let package = Package(
       ],
       swiftSettings: allTargetsSwiftSettings),
 
+    .executableTarget(
+      name: "hylo-demangle",
+      dependencies: [
+        "IR"
+      ],
+      swiftSettings: allTargetsSwiftSettings),
+
     .target(
       name: "Driver",
       dependencies: [

Sources/hylo-demangle/main.swift

@@ -0,0 +1,24 @@
+import Foundation
+import IR
+
+/// Reports the given `diagnostic` on the standard error and exit with status -1.
+func error(_ diagnostic: String) -> Never {
+  let d = Data("\(diagnostic)\n".utf8)
+  FileHandle.standardError.write(d)
+  exit(-1)
+}
+
+func main() {
+  guard CommandLine.arguments.count > 1 else {
+    error("missing input")
+  }
+
+  let n = CommandLine.arguments[1]
+  guard let s = DemangledSymbol(n) else {
+    error("could not demangle '\(n)'")
+  }
+
+  print(s)
+}
+
+main()