Fixup: Merge llvm-project release/18.x llvmorg-18.1.5-0-g617a15a9eac9

Update version numbers, config headers, etc. Git tricked me into losing
these before pushing.

PR: 276104
Approved by: re (cperciva)
Fixes: d67fc74b9249
MFC after: 3 days

(cherry picked from commit 894cb08f0d3656fdb81f4d89085bedc4235f3cb6)
(cherry picked from commit a17c251dd68567d879bb80e62f9828abb3967c64)