[Dependency Scanning] Do now write out bridging header dependencies of binary modules unless in CAS mode

We only record these dependencies in CAS mode, because we require explicit PCH tasks to be produced for imported header of binary module dependencies. In the meantime, in non-CAS mode loading clients will consume the `.h` files encoded in the `.swiftmodules` directly.

Followup changes to SwiftDriver will enable explicit PCH compilation of such dependenceis, but for the time being restore prior behavior for non-CAS explicit module builds.

Resolves rdar://116006619

Author: artemcm

lib/Serialization/SerializedModuleLoader.cpp

@@ -477,12 +477,19 @@ SerializedModuleLoaderBase::scanModuleFile(Twine modulePath, bool isFramework) {
 
   auto importedHeaderSet = binaryModuleImports.get().headerImports;
   std::vector<std::string> importedHeaders;
-  importedHeaders.reserve(importedHeaderSet.size());
-  llvm::transform(importedHeaderSet.keys(),
-                  std::back_inserter(importedHeaders),
-                  [](llvm::StringRef N) {
-                     return N.str();
-                  });
+  // FIXME: We only record these dependencies in CAS mode, because
+  // we require explicit PCH tasks to be produced for imported header
+  // of binary module dependencies. In the meantime, in non-CAS mode
+  // loading clients will consume the `.h` files encoded in the `.swiftmodules`
+  // directly.
+  if (Ctx.ClangImporterOpts.CASOpts) {
+    importedHeaders.reserve(importedHeaderSet.size());
+    llvm::transform(importedHeaderSet.keys(),
+                    std::back_inserter(importedHeaders),
+                    [](llvm::StringRef N) {
+      return N.str();
+    });
+  }
 
   auto &importedOptionalModuleSet = binaryModuleOptionalImports.get().moduleImports;
   std::vector<std::string> importedOptionalModuleNames;

test/ScanDependencies/header_deps_of_binary.swift

@@ -2,18 +2,20 @@
 // RUN: %empty-directory(%t)
 // RUN: %empty-directory(%t/clang-module-cache)
 // RUN: %empty-directory(%t/PCH)
+// RUN: %empty-directory(%t/HEADER)
 // RUN: %empty-directory(%t/SwiftModules)
+// RUN: %empty-directory(%t/CAS)
 
 // - Set up Foo Swift dependency
 // RUN: echo "extension Profiler {" >> %t/foo.swift
 // RUN: echo "    public static let count: Int = 42" >> %t/foo.swift
 // RUN: echo "}" >> %t/foo.swift
 
 // - Set up Foo bridging header
-// RUN: echo "struct Profiler { void* ptr; };" >> %t/foo.h
+// RUN: echo "struct Profiler { void* ptr; };" >> %t/HEADER/foo.h
 
 // - Compile bridging header
-// RUN: %target-swift-frontend -enable-objc-interop -emit-pch %t/foo.h -o %t/PCH/foo.pch -disable-implicit-swift-modules
+// RUN: %target-swift-frontend -enable-objc-interop -emit-pch %t/HEADER/foo.h -o %t/PCH/foo.pch -disable-implicit-swift-modules
 
 // - Set up explicit dependencies for Foo
 // RUN: %target-swift-emit-pcm -module-name SwiftShims %swift-lib-dir/swift/shims/module.modulemap -o %t/inputs/SwiftShims.pcm
@@ -50,21 +52,27 @@
 // RUN: %target-swift-frontend -emit-module -emit-module-path %t/SwiftModules/Foo.swiftmodule %t/foo.swift -module-name Foo -import-objc-header %t/PCH/foo.pch -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -disable-implicit-swift-modules -explicit-swift-module-map-file %t/foo_inputs_map.json 
 
 // - Scan main module
-// RUN: %target-swift-frontend -scan-dependencies %s -I %t/SwiftModules -I %S/Inputs/Swift -o %t/deps.json
+// RUN: %target-swift-frontend -scan-dependencies %s -I %t/SwiftModules -I %S/Inputs/Swift -o %t/deps.json -cache-compile-job -cas-path %t/cas
 // RUN: %validate-json %t/deps.json | %FileCheck %s
 
+// - Scan main module without a CAS and ensure no headerDependencies are emitted
+// RUN: %target-swift-frontend -scan-dependencies %s -I %t/SwiftModules -I %S/Inputs/Swift -o %t/deps_nocas.json
+// RUN: %validate-json %t/deps_nocas.json | %FileCheck %s --check-prefix=CHECK-NO-HEADERS
+
 // CHECK: "swift": "FooClient"
 // CHECK: "swift": "FooClient"
 // CHECK: "swiftPrebuiltExternal": "Foo"
 // CHECK:     "commandLine": [
 // CHECK:            "-include-pch",
 // CHECK-NEXT:       "-Xcc",
-// CHECK-NEXT:       "{{.*}}{{/|\\}}PCH{{/|\\}}foo.pch"
+// CHECK-NEXT:       "{{.*}}{{/|\\}}HEADER{{/|\\}}foo.h"
 
 
 // CHECK: "swiftPrebuiltExternal": "Foo"
 // CHECK:   "headerDependencies": [
-// CHECK:      "{{.*}}{{/|\\}}PCH{{/|\\}}foo.pch"
+// CHECK:      "{{.*}}{{/|\\}}HEADER{{/|\\}}foo.h"
 // CHECK:   ],
 
+// CHECK-NO-HEADERS-NOT: "headerDependencies"
+
 import FooClient