Core: add missing flags for Windows

Windows core modules require the `-fbuiltin-headers-in-system-modules`
flag to be passed to the clang importer. Adjust the build flags for the
platform.

Author: compnerd

Runtimes/Core/CMakeLists.txt

@@ -125,6 +125,7 @@ add_compile_options(
   $<$<COMPILE_LANGUAGE:CXX>:-fno-rtti>
   $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions>
   $<$<COMPILE_LANGUAGE:CXX>:-funwind-tables>
+  "$<$<AND:$<COMPILE_LANGUAGE:Swift>,$<PLATFORM_ID:Windows>>:SHELL:-Xcc -Xclang -Xcc -fbuiltin-headers-in-system-modules>"
   $<$<AND:$<COMPILE_LANGUAGE:Swift>,$<BOOL:${SwiftCore_ENABLE_LIBRARY_EVOLUTION}>>:-enable-library-evolution>)
 
 include_directories(include)