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

This reverts commit c6ce6ee.

Author: hamishknight

SwiftCompilerSources/CMakeLists.txt

@@ -101,12 +101,19 @@ function(add_swift_compiler_modules_library name)
   set(swift_compile_options
       "-color-diagnostics"
       "-Xfrontend" "-validate-tbd-against-ir=none"
-      "-cxx-interoperability-mode=default"
+      "${cxx_interop_flag}"
       "-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
@@ -334,7 +341,7 @@ 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 5.9)
+      set(min_supported_swift_version 5.8)
       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             "Outdated Swift compiler: building with host tools requires Swift ${min_supported_swift_version} or newer. "