ci: Use newer LLVM version with gcc, to avoid compiler warnings

gcc emits a warning for LLVM 14 code outside of our control. To avoid that,
update to a newer LLVM version. Do so both in the CompilerWarnings and normal
tasks - the latter don't fail, but the warnings make it more likely that we'd
miss other warnings.

We might want to backpatch this eventually. The higher priority right now is
to unbreak CI though - which is only broken on master, due to 0c3930d076
interacting badly with c8a6ec206a (mea culpa, I should have noticed this
before pushing, but I missed it due to another, independent CI failure).

Discussion: https://postgr.es/m/20240715193754.awdxgrzurxnwwu2t@awork3.anarazel.de

.cirrus.tasks.yml

@@ -272,6 +272,8 @@ task:
     LDFLAGS: $SANITIZER_FLAGS
     CC: ccache gcc
     CXX: ccache g++
+    # GCC emits a warning for llvm-14, so switch to a newer one.
+    LLVM_CONFIG: llvm-config-16
 
     LINUX_CONFIGURE_FEATURES: *LINUX_CONFIGURE_FEATURES
     LINUX_MESON_FEATURES: *LINUX_MESON_FEATURES
@@ -334,7 +336,7 @@ task:
             \
             ${LINUX_CONFIGURE_FEATURES} \
             \
-            CLANG="ccache clang"
+            CLANG="ccache clang-16"
         EOF
       build_script: su postgres -c "make -s -j${BUILD_JOBS} world-bin"
       upload_caches: ccache
@@ -661,6 +663,9 @@ task:
     LINUX_CONFIGURE_FEATURES: *LINUX_CONFIGURE_FEATURES
     LINUX_MESON_FEATURES: *LINUX_MESON_FEATURES
 
+    # GCC emits a warning for llvm-14, so switch to a newer one.
+    LLVM_CONFIG: llvm-config-16
+
   <<: *linux_task_template
 
   sysinfo_script: |
@@ -696,7 +701,7 @@ task:
         --cache gcc.cache \
         --enable-dtrace \
         ${LINUX_CONFIGURE_FEATURES} \
-        CC="ccache gcc" CXX="ccache g++" CLANG="ccache clang"
+        CC="ccache gcc" CXX="ccache g++" CLANG="ccache clang-16"
       make -s -j${BUILD_JOBS} clean
       time make -s -j${BUILD_JOBS} world-bin
 
@@ -707,7 +712,7 @@ task:
         --cache gcc.cache \
         --enable-cassert \
         ${LINUX_CONFIGURE_FEATURES} \
-        CC="ccache gcc" CXX="ccache g++" CLANG="ccache clang"
+        CC="ccache gcc" CXX="ccache g++" CLANG="ccache clang-16"
       make -s -j${BUILD_JOBS} clean
       time make -s -j${BUILD_JOBS} world-bin
 
@@ -717,7 +722,7 @@ task:
       time ./configure \
         --cache clang.cache \
         ${LINUX_CONFIGURE_FEATURES} \
-        CC="ccache clang" CXX="ccache clang++" CLANG="ccache clang"
+        CC="ccache clang" CXX="ccache clang++-16" CLANG="ccache clang-16"
       make -s -j${BUILD_JOBS} clean
       time make -s -j${BUILD_JOBS} world-bin
 
@@ -729,7 +734,7 @@ task:
         --enable-cassert \
         --enable-dtrace \
         ${LINUX_CONFIGURE_FEATURES} \
-        CC="ccache clang" CXX="ccache clang++" CLANG="ccache clang"
+        CC="ccache clang" CXX="ccache clang++-16" CLANG="ccache clang-16"
       make -s -j${BUILD_JOBS} clean
       time make -s -j${BUILD_JOBS} world-bin
 
@@ -753,9 +758,7 @@ task:
     docs_build_script: |
       time ./configure \
         --cache gcc.cache \
-        CC="ccache gcc" \
+        CC="ccache gcc" CXX="ccache g++" CLANG="ccache clang-16"
-        CXX="ccache g++" \
-        CLANG="ccache clang"
       make -s -j${BUILD_JOBS} clean
       time make -s -j${BUILD_JOBS} -C doc
 
@@ -774,7 +777,7 @@ task:
         ${LINUX_CONFIGURE_FEATURES} \
         --without-icu \
         --quiet \
-        CC="gcc" CXX"=g++" CLANG="clang"
+        CC="gcc" CXX"=g++" CLANG="clang-16"
       make -s -j${BUILD_JOBS} clean
       time make -s headerscheck EXTRAFLAGS='-fmax-errors=10'
     headers_cpluspluscheck_script: |