Cirrus CI: Adapt to FreeBSD bumping default LLVM from 15 to 19 (#4865)

The lit-tests fail with LLVM 19 and 18 (PGO and sanitizer issues), so sticking with LLVM 15.

Author: kinke

.cirrus.yml

@@ -220,13 +220,17 @@ task:
       -DD_COMPILER_FLAGS="-O -flto=full -defaultlib=phobos2-ldc-lto,druntime-ldc-lto"
       -DEXTRA_CXXFLAGS=-flto=full
     PARALLELISM: 4
-    CC: clang
+    CC: clang15
+    CXX: clang++15
   install_prerequisites_script: |
     cd $CIRRUS_WORKING_DIR/..
     sysctl -n hw.ncpu
-    pkg install -y git cmake ninja gmake llvm bash gtar 7-zip ldc
+    pkg install -y git cmake ninja gmake llvm15 bash gtar 7-zip ldc
     python3 --version
     python3 -m ensurepip
+    # set up default llvm-config
+    ls -l /usr/local/bin/llvm-config*
+    ln -sf llvm-config15 /usr/local/bin/llvm-config
   clone_submodules_early_script: |
     cd $CIRRUS_WORKING_DIR
     git submodule update --init --depth $CIRRUS_CLONE_DEPTH
@@ -239,6 +243,7 @@ task:
       -DCMAKE_BUILD_TYPE=Release \
       -DD_COMPILER=ldmd2 \
       -DBUILD_SHARED_LIBS=OFF \
+      -DLDC_DYNAMIC_COMPILE=OFF \
       -DBUILD_LTO_LIBS=ON
     ninja -j$PARALLELISM
     bin/ldc2 -version