[SwiftCompilerSources] Require very recent Swift compiler in HOSTTOOLS mode

To use new C++ interop features in SwiftCompilerSources, we need a very recent Swift compiler when building with host tools.

This does not affect non-HOSTTOOLS builds.

Author: egorzhdan

SwiftCompilerSources/CMakeLists.txt

@@ -209,7 +209,7 @@ 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 5.5)
+      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             "Outdated Swift compiler: building with host tools requires Swift ${min_supported_swift_version} or newer. "