[Explicit Module Builds] Pass in -fno-implicit-module-maps for pre-building Clang explicit dependencies

The dependency scanner specifies which exact modulemaps must be used, we do not need Clang to also search for modulemaps.

Author: artemcm

Sources/SwiftDriver/ExplicitModuleBuilds/ExplicitDependencyBuildPlanner.swift

@@ -254,7 +254,7 @@ public typealias ExternalTargetModuleDetailsMap = [ModuleDependencyId: ExternalT
                                                           commandLine: inout [Job.ArgTemplate]) throws {
     // Prohibit the frontend from implicitly building textual modules into binary modules.
     commandLine.appendFlags("-disable-implicit-swift-modules", "-Xcc", "-Xclang", "-Xcc",
-                            "-fno-implicit-modules")
+                            "-fno-implicit-modules", "-Xcc", "-Xclang", "-Xcc", "-fno-implicit-module-maps")
     var swiftDependencyArtifacts: [SwiftModuleArtifactInfo] = []
     var clangDependencyArtifacts: [ClangModuleArtifactInfo] = []
     try addModuleDependencies(moduleId: moduleId, pcmArgs: pcmArgs,

Tests/SwiftDriverTests/ExplicitModuleBuildTests.swift

@@ -59,6 +59,8 @@ throws {
   // Ensure the frontend was prohibited from doing implicit module builds
   XCTAssertTrue(job.commandLine.contains(.flag(String("-disable-implicit-swift-modules"))))
   XCTAssertTrue(job.commandLine.contains(.flag(String("-fno-implicit-modules"))))
+  XCTAssertTrue(job.commandLine.contains(.flag(String("-fno-implicit-module-maps"))))
+
   try checkExplicitModuleBuildJobDependencies(job: job, pcmArgs: downstreamPCMArgs,
                                               moduleInfo: moduleInfo,
                                               dependencyGraph: dependencyGraph,