[SwiftCompilerSources] Build without target OS checking

Temporarily workaround an availability issue with CxxStdlib when
building with --bootstrapping=hosttools.

Author: rintaro

SwiftCompilerSources/CMakeLists.txt

@@ -75,6 +75,7 @@ function(add_swift_compiler_modules_library name)
   set(swift_compile_options
       "-Xfrontend" "-validate-tbd-against-ir=none"
       "-Xfrontend" "-enable-experimental-cxx-interop"
+      "-Xfrontend" "-disable-target-os-checking"
       "-Xcc" "-std=c++17"
       "-Xcc" "-DCOMPILED_WITH_SWIFT"
       "-Xcc" "-UIBOutlet" "-Xcc" "-UIBAction" "-Xcc" "-UIBInspectable")