Cirrus-CI: bump Clang versions to 17 and 18

Clang/LLVM 17 is currently the in-tree default compiler, and Clang/LLVM
18 is coming soon. Use the former as the default CI toolchain and make
the latter available for manual invocation.

Sponsored by:	The FreeBSD Foundation
Differential Revision:

.cirrus.yml

@@ -14,36 +14,36 @@ env:
 
 task:
   matrix:
-  - name: amd64-llvm15 World and kernel build and boot smoke test
-    only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
-    trigger_type: manual
+  - name: amd64-llvm17 World and kernel build and boot smoke test
+    only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src' || $CIRRUS_BRANCH =~ 'pull/.*'
     env:
       TARGET: amd64
       TARGET_ARCH: amd64
-      TOOLCHAIN: llvm15
+      TOOLCHAIN: llvm17
       TOOLCHAIN_PKG: ${TOOLCHAIN}-lite
-  - name: amd64-llvm16 World and kernel build and boot smoke test
-    only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src' || $CIRRUS_BRANCH =~ 'pull/.*'
+  - name: amd64-llvm18 World and kernel build and boot smoke test
+    only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
+    trigger_type: manual
     env:
       TARGET: amd64
       TARGET_ARCH: amd64
-      TOOLCHAIN: llvm16
+      TOOLCHAIN: llvm18
       TOOLCHAIN_PKG: ${TOOLCHAIN}-lite
-  - name: arm64-llvm15 World and kernel build and boot smoke test
+  - name: arm64-llvm17 World and kernel build and boot smoke test
     only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
     trigger_type: manual
     env:
       TARGET: arm64
       TARGET_ARCH: aarch64
-      TOOLCHAIN: llvm15
+      TOOLCHAIN: llvm17
       TOOLCHAIN_PKG: ${TOOLCHAIN}
-  - name: arm64-llvm16 World and kernel build and boot smoke test
+  - name: arm64-llvm18 World and kernel build and boot smoke test
     only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
     trigger_type: manual
     env:
       TARGET: arm64
       TARGET_ARCH: aarch64
-      TOOLCHAIN: llvm16
+      TOOLCHAIN: llvm18
       TOOLCHAIN_PKG: ${TOOLCHAIN}
   - name: amd64-gcc12 World and kernel build and boot smoke test (manual)
     only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'