[cxx-interop] Remove CxxStdlib dependency on Darwin

This was causing build errors when building the compiler and the stdlib separately.

rdar://107290998

Author: egorzhdan

stdlib/public/Cxx/std/CMakeLists.txt

@@ -132,11 +132,6 @@ add_swift_target_library(swiftCxxStdlib STATIC NO_LINK_NAME IS_STDLIB
     String.swift
 
     SWIFT_MODULE_DEPENDS Cxx
-    SWIFT_MODULE_DEPENDS_OSX Darwin
-    SWIFT_MODULE_DEPENDS_IOS Darwin
-    SWIFT_MODULE_DEPENDS_TVOS Darwin
-    SWIFT_MODULE_DEPENDS_WATCHOS Darwin
-    SWIFT_MODULE_DEPENDS_LINUX Glibc
 
     SWIFT_COMPILE_FLAGS ${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS} ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     -Xfrontend -enable-experimental-cxx-interop