Adjust to the new clang builtin modules

clang is adding builtin modules for its C standard library headers. This requires proper module layering with the OS/SDK headers, but Backtracing is currently just absorbing them all into its OS module. Pass `-Xclang -fbuiltin-headers-in-system-modules` to allow that to continue working.

Explicitly include <stddef.h> so that it gets exported from the OS module.

Author: ian-twilightcoder

stdlib/public/Backtracing/CMakeLists.txt

@@ -51,6 +51,7 @@ set(BACKTRACING_COMPILE_FLAGS
   "-Xcc;-I${CMAKE_BINARY_DIR}/include"
   "-Xcc;-fno-implicit-module-maps"
   "-Xcc;-fbuiltin-module-map"
+  "-Xcc;-Xclang;-Xcc;-fbuiltin-headers-in-system-modules"
   "-Xcc;-fmodule-map-file=${SWIFT_STDLIB_SOURCE_DIR}/public/SwiftShims/swift/shims/module.modulemap"
   "-Xcc;-fmodule-map-file=${SWIFT_STDLIB_SOURCE_DIR}/public/Backtracing/modules/module.modulemap")
 

stdlib/public/Backtracing/modules/OS/Linux.h

@@ -19,6 +19,7 @@
 #ifdef __linux__
 
 #define _GNU_SOURCE
+#include <stddef.h>
 #include <sys/uio.h>
 #include <ucontext.h>