[SwiftCompilerSources][build] Bump the minimum supported compiler version to 5.9

Building SwiftCompilerSources with a Swift 5.8 compiler is not tested anymore, and it might showcase bugs in the old compiler version that are already fixed.

This bumps the minimum allowed version to Swift 5.9, which was shipped in Xcode 15.

Author: egorzhdan

SwiftCompilerSources/CMakeLists.txt

@@ -101,19 +101,12 @@ function(add_swift_compiler_modules_library name)
   set(swift_compile_options
       "-color-diagnostics"
       "-Xfrontend" "-validate-tbd-against-ir=none"
-      "${cxx_interop_flag}"
+      "-cxx-interoperability-mode=default"
       "-Xfrontend" "-disable-target-os-checking"
       "-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)
       list(APPEND swift_compile_options
@@ -327,7 +320,7 @@ else()
         message(FATAL_ERROR "The Swift compiler (${CMAKE_Swift_COMPILER}) differs from the Swift compiler in SWIFT_NATIVE_SWIFT_TOOLS_PATH (${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/swiftc).")
       endif()
 
-      set(min_supported_swift_version 5.8)
+      set(min_supported_swift_version 5.9)
       if(CMAKE_Swift_COMPILER_VERSION VERSION_LESS "${min_supported_swift_version}")
         message(FATAL_ERROR
             "Outdated Swift compiler: building with host tools requires Swift ${min_supported_swift_version} or newer. "