[StdlibUnittest] Avoid serializing references to ObjC runtime APIs.

Specifically, objc_setUncaughtExceptionHandler, whose nullability is
now annotated in Swift 4 mode when using the Xcode 9 SDKs. That means
it has a different signature in Swift 3 and Swift 4 mode, which means
that references from SIL will fail when deserialized in the other
mode. (The recovery support added in earlier commits doesn't cover
serialized SIL.)

This resulted in crashes when building with
-build-serialized-stdlib-unittest, which enables -sil-serialize-all /
"magic performance mode" when building the StdlibUnittest module like
it does the standard libarry.

None of this affects developer code, because developer code doesn't
serialize anything (if they're playing by the rules).

Author: jrose-apple

stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb

@@ -624,6 +624,8 @@ func _stdlib_installTrapInterceptor()
 }
 #endif
 
+// Avoid serializing references to objc_setUncaughtExceptionHandler in SIL.
+@inline(never) @_semantics("stdlib_binary_only")
 func _childProcess() {
   _stdlib_installTrapInterceptor()