Add UPDATING note about running make delete-old after libc++ 18 upgrade

PR:		279692
MFC after:	3 days

1 files changed, 7 insertions, 0 deletions

diff --git a/UPDATING b/UPDATING
index cffafdb5d247..1aae85df0a70 100644
--- a/UPDATING
+++ b/UPDATING
@@ -55,6 +55,13 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 15.x IS SLOW:
 	own packages.  If you use pkgbase, you should install the relevant
 	packages: FreeBSD-cron, FreeBSD-lp, or FreeBSD-ntp.
 
+20240406:
+	Clang, llvm, lld, lldb, compiler-rt, libc++, libunwind and openmp have
+	been upgraded to 18.1.6. It is important that you run `make delete-old`
+	as described in the COMMON ITEMS section, otherwise several libc++
+	headers that are obsolete and need to be removed can cause compilation
+	errors in C++ programs.
+
 20240205:
 	For dynamically linked programs, system calls are now made from
 	libsys rather than libc.  No change in linkage is required as