Add additional implicit framework search path for Darwin platforms

'/System/Library/SubFrameworks' will be searched for frameworks by the compiler implicitly when an SDK path is specified.

Resolves rdar://137454957

Author: artemcm

include/swift/AST/SearchPathOptions.h

@@ -405,9 +405,13 @@ class SearchPathOptions {
     SmallString<128> systemFrameworksScratch(NewSDKPath);
     llvm::sys::path::append(systemFrameworksScratch, "System", "Library",
                             "Frameworks");
+    SmallString<128> systemSubFrameworksScratch(NewSDKPath);
+    llvm::sys::path::append(systemSubFrameworksScratch, "System", "Library",
+                            "SubFrameworks");
     SmallString<128> frameworksScratch(NewSDKPath);
     llvm::sys::path::append(frameworksScratch, "Library", "Frameworks");
     DarwinImplicitFrameworkSearchPaths = {systemFrameworksScratch.str().str(),
+                                          systemSubFrameworksScratch.str().str(),
                                           frameworksScratch.str().str()};
 
     Lookup.searchPathsDidChange();

test/ScanDependencies/subframework_implicit_search_path.swift

@@ -0,0 +1,23 @@
+// REQUIRES: objc_interop
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/module-cache)
+// RUN: %empty-directory(%t/mock.sdk/System/Library)
+
+// RUN: %empty-directory(%t/mock.sdk/System/Library/Frameworks)
+// RUN: %empty-directory(%t/mock.sdk/System/Library/SubFrameworks)
+
+// RUN: %empty-directory(%t/mock.sdk/System/Library/Frameworks/E.framework/Modules/E.swiftmodule)
+// RUN: %empty-directory(%t/mock.sdk/System/Library/SubFrameworks/SubE.framework/Modules/SubE.swiftmodule)
+
+// RUN: cp %S/Inputs/Swift/E.swiftinterface %t/mock.sdk/System/Library/Frameworks/E.framework/Modules/E.swiftmodule/%target-swiftinterface-name
+// RUN: cp %S/Inputs/Swift/SubE.swiftinterface %t/mock.sdk/System/Library/SubFrameworks/SubE.framework/Modules/SubE.swiftmodule/%target-swiftinterface-name
+
+// RUN: %target-swift-frontend -scan-dependencies -module-cache-path %t/module-cache %s -o %t/deps.json -sdk %t/mock.sdk
+// RUN: %validate-json %t/deps.json | %FileCheck %s
+
+import E
+import SubE
+
+// CHECK: "mainModuleName": "deps"
+// CHECK: "swift": "E"
+// CHECK: "swift": "SubE"