Remove references to obsolete '.swiftPlaceholder' module type. This is a leftover from a prior design which had Swift Build specify paths to to-be-built modules to the driver. This designed turned out to be not viable for multiple reasons and was abandoned a couple of years ago. We are now workgin towards removing this code.

Author: artemcm

Sources/SWBCore/LibSwiftDriver/LibSwiftDriver.swift

@@ -200,7 +200,9 @@ public final class SwiftModuleDependencyGraph: SwiftGlobalExplicitDependencyGrap
                 if let modulePath = VirtualPath.lookup(details.compiledModulePath.path).absolutePath {
                     swiftmodulePaths.append(modulePath.pathString)
                 }
-            case .clang, .swiftPlaceholder:
+            case .clang:
+                fallthrough
+            default:
                 break
             }
         }
@@ -223,15 +225,13 @@ public final class SwiftModuleDependencyGraph: SwiftGlobalExplicitDependencyGrap
             }
             fileDependencies.append(contentsOf: moduleInfo.sourceFiles ?? [])
             switch moduleInfo.details {
-            case .swift:
-                break
-            case .swiftPlaceholder:
-                break
             case .swiftPrebuiltExternal(let details):
                 if let modulePath = VirtualPath.lookup(details.compiledModulePath.path).absolutePath {
                     fileDependencies.append(modulePath.pathString)
                 }
-            case .clang:
+            case .swift, .clang:
+                fallthrough
+            default:
                 break
             }
         }