[Explicit Modules] Pass '-explicit-interface-module-build' to interface build jobs.

Sources/SwiftDriver/ExplicitModuleBuilds/ExplicitDependencyBuildPlanner.swift

@@ -50,14 +50,19 @@ public typealias ExternalTargetModuleDetailsMap = [ModuleDependencyId: ExternalT
   /// We avoid re-running the hash computation with the use of this cache
   private var hashedModuleNameCache: [String: String] = [:]
 
+  /// Does this compile support `.explicitInterfaceModuleBuild`
+  private var supportsExplicitInterfaceBuild: Bool
+
   public init(dependencyGraph: InterModuleDependencyGraph,
               toolchain: Toolchain,
-              integratedDriver: Bool = true) throws {
+              integratedDriver: Bool = true,
+              supportsExplicitInterfaceBuild: Bool = false) throws {
     self.dependencyGraph = dependencyGraph
     self.toolchain = toolchain
     self.integratedDriver = integratedDriver
     self.mainModuleName = dependencyGraph.mainModuleName
     self.reachabilityMap = try dependencyGraph.computeTransitiveClosure()
+    self.supportsExplicitInterfaceBuild = supportsExplicitInterfaceBuild
   }
 
   /// Generate build jobs for all dependencies of the main module.
@@ -165,6 +170,12 @@ public typealias ExternalTargetModuleDetailsMap = [ModuleDependencyId: ExternalT
         }
       }
 
+      if supportsExplicitInterfaceBuild {
+        // Ensure the compiler flags specified in the interface are ignored
+        // because they are already captured in the dependency scanner output
+        commandLine.appendFlag(.explicitInterfaceModuleBuild)
+      }
+
       // Set the output path
       commandLine.appendFlag(.o)
       commandLine.appendPath(VirtualPath.lookup(moduleInfo.modulePath.path))

Sources/SwiftDriver/Jobs/Planning.swift

@@ -626,7 +626,9 @@ extension Driver {
     explicitDependencyBuildPlanner =
       try ExplicitDependencyBuildPlanner(dependencyGraph: dependencyGraph,
                                          toolchain: toolchain,
-                                         integratedDriver: integratedDriver)
+                                         integratedDriver: integratedDriver,
+                                         supportsExplicitInterfaceBuild:
+                                          isFrontendArgSupported(.explicitInterfaceModuleBuild))
 
     return try explicitDependencyBuildPlanner!.generateExplicitModuleDependenciesBuildJobs()
   }