[gn/mac] bump HOST_LINK_VERSION to 520

With this, clang will pass -platform_version instead of
-mmacosx_version_min to the linker. Recent versions of the linker
complain that the flag is now spelled mmacos_version_min (without
the x), and this supresses that warning.

520 is over 4 years old by now, so just changing this unconditionally
seems fine.

llvm/utils/gn/secondary/clang/include/clang/Config/BUILD.gn

diff --git a/llvm/utils/gn/secondary/clang/include/clang/Config/BUILD.gn b/llvm/utils/gn/secondary/clang/include/clang/Config/BUILD.gn
index a3fb952..bc0631d 100644
--- a/llvm/utils/gn/secondary/clang/include/clang/Config/BUILD.gn
+++ b/llvm/utils/gn/secondary/clang/include/clang/Config/BUILD.gn
@@ -68,7 +68,7 @@
     # FIXME: Hardcoding this isn't great, but assuming that the host ld version
     # has anything to do with the ld version where the built clang will run
     # isn't either. Probably want to make this a declare_args.
-    values += [ "HOST_LINK_VERSION=305" ]
+    values += [ "HOST_LINK_VERSION=520" ]
   } else {
     values += [ "HOST_LINK_VERSION=" ]
   }