Prefix experimental interop flag with -Xfrontend

(cherry picked from commit 40f8cfd)

Author: adrian-prantl

SwiftCompilerSources/CMakeLists.txt

@@ -98,13 +98,6 @@ function(add_swift_compiler_modules_library name)
                         "DEPENDS"
                         ${ARGN})
 
-  # Prior to 5.9, we have to use the experimental flag for C++ interop.
-  if (CMAKE_Swift_COMPILER_VERSION VERSION_LESS 5.9)
-    set(cxx_interop_flag "-enable-experimental-cxx-interop")
-  else()
-    set(cxx_interop_flag "-cxx-interoperability-mode=default")
-  endif()
-
   set(swift_compile_options
       "-color-diagnostics"
       "-Xfrontend" "-validate-tbd-against-ir=none"
@@ -113,6 +106,12 @@ function(add_swift_compiler_modules_library name)
       "-Xcc" "-std=c++17"
       "-Xcc" "-DCOMPILED_WITH_SWIFT" "-Xcc" "-DSWIFT_TARGET"
       "-Xcc" "-UIBOutlet" "-Xcc" "-UIBAction" "-Xcc" "-UIBInspectable")
+  # Prior to 5.9, we have to use the experimental flag for C++ interop.
+  if (CMAKE_Swift_COMPILER_VERSION VERSION_LESS 5.9)
+    list(APPEND swift_compile_options "-Xfrontend" "-enable-experimental-cxx-interop")
+  else()
+    list(APPEND swift_compile_options "-cxx-interoperability-mode=default")
+  endif()
 
   if (NOT BOOTSTRAPPING_MODE STREQUAL "HOSTTOOLS")
     if(SWIFT_MIN_RUNTIME_VERSION)

lib/ASTGen/CMakeLists.txt

@@ -94,17 +94,9 @@ foreach(c_include_path ${c_include_paths})
   list(APPEND c_include_paths_args "SHELL: -Xcc -I -Xcc ${c_include_path}")
 endforeach()
 
-# Prior to 5.9, we have to use the experimental flag for C++ interop.
-if (CMAKE_Swift_COMPILER_VERSION VERSION_LESS 5.9)
-  set(cxx_interop_flag "-enable-experimental-cxx-interop")
-else()
-  set(cxx_interop_flag "-cxx-interoperability-mode=default")
-endif()
-
 set(compile_options
   ${c_include_paths_args}
   "SHELL: -DRESILIENT_SWIFT_SYNTAX"
-  "SHELL: ${cxx_interop_flag}"
   "SHELL: -Xcc -std=c++17 -Xcc -DCOMPILED_WITH_SWIFT"
 
   # FIXME: Needed to work around an availability issue with CxxStdlib
@@ -116,6 +108,13 @@ set(compile_options
   "SHELL:-Xcc -D_CRT_USE_BUILTIN_OFFSETOF"
 )
 
+# Prior to 5.9, we have to use the experimental flag for C++ interop.
+if (CMAKE_Swift_COMPILER_VERSION VERSION_LESS 5.9)
+  list(APPEND compile_options "-Xfrontend" "-enable-experimental-cxx-interop")
+else()
+  list(APPEND compile_options "-cxx-interoperability-mode=default")
+endif()
+
 if(SWIFT_BUILD_SWIFT_SYNTAX)
   foreach(target swiftASTGen swiftLLVMJSON swiftIDEUtilsBridging)
     target_compile_options(${target} PRIVATE ${compile_options})