renderdiff: fix llvm-18 dep problem (#8347)

The public dep for mesa has moved on to beyond llvm-18, but the
version we're compiling against is not supported by this change.
We workaround by ensuring that the llvm dep is less than 18 but
consistent with the clang version.

test/utils/get_mesa.sh

@@ -17,8 +17,21 @@
 set -e
 set -x
 
+CLANG_VERSION=`clang --version | head -n 1 | awk '{ print $4 }' | python3 -c "import sys; print(sys.stdin.read().split('.')[0])"`
+
 sudo apt-get -y build-dep mesa
 
+# Uninstall llvm-18 (and above) as they conflict with the mesa version we are compiling.
+sudo apt -y remove llvm-18 llvm-18-*
+
+sudo apt -y install clang-${CLANG_VERSION} \
+     libc++-${CLANG_VERSION}-dev \
+     libc++abi-${CLANG_VERSION}-dev \
+     llvm-${CLANG_VERSION} \
+     llvm-${CLANG_VERSION}-{dev,tools,runtime}
+
+export CXX=`which clang++` && export CC=`which clang`
+
 git clone https://gitlab.freedesktop.org/mesa/mesa.git
 
 pushd .