[SwiftCompilerSources][build] Require recent Swift compiler in HOSTTOOLS mode

Some code in SwiftCompilerSources, especially the parts that use C++ interop, triggers bugs in older versions of the Swift compiler. To make sure that `HOSTTOOLS` builds don't produce obscure compiler errors or crashes, let's require a relatively new Swift compiler for `HOSTTOOLS` build.

Author: egorzhdan

SwiftCompilerSources/CMakeLists.txt

@@ -203,6 +203,12 @@ else()
     if (NOT CMAKE_Swift_COMPILER)
       message(FATAL_ERROR "Need a swift toolchain building swift compiler sources")
     endif()
+    set(min_supported_swift_version 5.5)
+    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+          "Outdated Swift compiler: building with host tools requires Swift ${min_supported_swift_version} or newer. "
+          "Please update your Swift toolchain or switch BOOTSTRAPPING_MODE to BOOTSTRAPPING(-WITH-HOSTLIBS)? or OFF.")
+    endif()
 
     add_swift_compiler_modules_library(swiftCompilerModules
       SWIFT_EXEC "${CMAKE_Swift_COMPILER}")