[windows] Allow compiler and STL version mismatch

VS 17.10 introduced a requirement on Clang 17, but the pinned Swift
toolchain used to bootstrap the compilers (a 5.10 snapshot) uses Clang 16. We
can relax this version requirement with ALLOW_COMPILER_AND_STL_VERSION_MISMATCH
which needs to be set for all CMAKE_Swift_FLAGS when building the
compilers, as well as explicitly in SwiftCompilerSources as the
invocation of swiftc there is via custom_command

Author: z2oh

SwiftCompilerSources/CMakeLists.txt

@@ -181,6 +181,13 @@ function(add_swift_compiler_modules_library name)
     # Workaround a crash in the LoadableByAddress pass
     # https://github.com/apple/swift/issues/73254
     list(APPEND swift_compile_options "-Xllvm" "-sil-disable-pass=loadable-address")
+
+    # The STL in VS 17.10 requires Clang 17 or higher, but bootstrapping generally uses toolchains with older versions
+    # versions of Clang. Swift 6 toolchains are the first to include Clang 17, so if we are on Windows and using an
+    # earlier toolchain, we need to relax to relax this requirement with ALLOW_COMPILER_AND_STL_VERSION_MISMATCH.
+    if (CMAKE_Swift_COMPILER_VERSION VERSION_LESS 6.0)
+      list(APPEND swift_compile_options "-Xcc" "-D_ALLOW_COMPILER_AND_STL_VERSION_MISMATCH")
+    endif()
   else()
     list(APPEND sdk_option "-I" "${swift_exec_bin_dir}/../lib" "-I" "${sdk_path}/usr/lib")
   endif()

utils/build.ps1

@@ -1323,6 +1323,7 @@ function Build-Compilers() {
       -Defines ($TestingDefines + @{
         CLANG_TABLEGEN = (Join-Path -Path $BuildTools -ChildPath "clang-tblgen.exe");
         CLANG_TIDY_CONFUSABLE_CHARS_GEN = (Join-Path -Path $BuildTools -ChildPath "clang-tidy-confusable-chars-gen.exe");
+        CMAKE_Swift_FLAGS = @("-Xcc", "-D_ALLOW_COMPILER_AND_STL_VERSION_MISMATCH");
         LLDB_PYTHON_EXE_RELATIVE_PATH = "python.exe";
         LLDB_PYTHON_EXT_SUFFIX = ".pyd";
         LLDB_PYTHON_RELATIVE_PATH = "lib/site-packages";