dev-lang/swift: fix building when dev-libs/libdispatch is present

Swift currently fails to build if `dev-libs/libdispatch` is installed
because it globally installs private `libdispatch` headers which get
unexpectedly picked up during the Swift build. Since the headers are not
expected to be present when building Swift on non-Darwin platforms, the
check for the header can be patched out.

Signed-off-by: Itai Ferber <itai@itaiferber.net>

dev-lang/swift/files/swift-6.2.4/disable-libdispatch-private-header-check.patch

@@ -0,0 +1,23 @@
+# `DispatchGlobalExecutor` checks for the presence of `dispatch/private.h` for
+# certain low-level libdispatch functionality, and because the header is
+# normally only available on Darwin platforms (it is never installed as part of
+# a normal Swift build on any platform), makes certain assumptions about
+# symbols available there. However, `dev-libs/libdispatch` explicitly installs
+# private headers system-wide for `gnustep-base/libobjc2`, and those get picked
+# up during the Swift build. Because of the structure of the header on Linux,
+# anyone who has `dev-libs/libdispatch` installed will fail to build.
+
+--- a/swift/stdlib/public/Concurrency/DispatchGlobalExecutor.cpp
++++ b/swift/stdlib/public/Concurrency/DispatchGlobalExecutor.cpp
+@@ -46,11 +46,6 @@
+ #endif
+ #endif
+ 
+-#if __has_include(<dispatch/private.h>)
+-#include <dispatch/private.h>
+-#define SWIFT_CONCURRENCY_HAS_DISPATCH_PRIVATE 1
+-#endif
+-
+ #include "Error.h"
+ #include "ExecutorImpl.h"
+ #include "TaskPrivate.h"