Add new flag to XCTest overlay

Author: Harlan Haskins

stdlib/public/Darwin/XCTest/CMakeLists.txt

@@ -7,7 +7,14 @@ add_swift_target_library(swiftXCTest ${SWIFT_SDK_OVERLAY_LIBRARY_BUILD_TYPES} IS
   XCTestCaseAdditions.mm
   XCTest.swift
 
-  SWIFT_COMPILE_FLAGS "${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS}"
+  SWIFT_COMPILE_FLAGS
+    "${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS}"
+
+    # XCTest has a type called XCTest, which exposes an issue in module
+    # interfaces -- because types are fully-qualified, the compiler currently
+    # doesn't disambiguate between XCTest-the-module and XCTest-the-class.
+    # rdar://48445154
+    -Xfrontend -module-interface-preserve-types-as-written
   LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
   TARGET_SDKS OSX IOS IOS_SIMULATOR TVOS TVOS_SIMULATOR
   SWIFT_MODULE_DEPENDS ObjectiveC Foundation