[CI] Bumping the CI image to support slang builds

Both clang-16 and gcc-11 build slang properly

circt-ci-build/Dockerfile

@@ -13,21 +13,31 @@ RUN apt-get update && apt-get install -y \
 
 RUN pip3 install yapf toml GitPython
 
-# Install latest release of LLVM
+# Install a more recent release of LLVM
 RUN wget https://apt.llvm.org/llvm.sh; \
-    chmod +x llvm.sh; \
-    ./llvm.sh 13;\
-    apt install -y clang-format-13 clang-tidy-13
-
-RUN ln -s /usr/bin/clang-13 /usr/bin/clang;\
-    ln -s /usr/bin/clang++-13 /usr/bin/clang++;\
-    ln -s /usr/bin/clang-tidy-13 /usr/bin/clang-tidy;\
-    ln -s /usr/bin/clang-tidy-diff-13.py /usr/bin/clang-tidy-diff;\
-    ln -s /usr/bin/clang-format-13 /usr/bin/clang-format;\
-    ln -s /usr/bin/clang-format-diff-13 /usr/bin/clang-format-diff;\
-    ln -s /usr/bin/git-clang-format-13 /usr/bin/git-clang-format;\
-    ln -s /usr/bin/lld-13 /usr/bin/lld;\
-    ln -s /usr/bin/lld-13 /usr/bin/ld.lld
+  chmod +x llvm.sh; \
+  ./llvm.sh 13;\
+  apt install -y clang-format-13 clang-tidy-13
+
+RUN ln -s /usr/bin/clang-13 /usr/bin/clang; \
+  ln -s /usr/bin/clang++-13 /usr/bin/clang++; \
+  ln -s /usr/bin/clang-tidy-13 /usr/bin/clang-tidy; \
+  ln -s /usr/bin/clang-tidy-diff-13.py /usr/bin/clang-tidy-diff; \
+  ln -s /usr/bin/clang-format-13 /usr/bin/clang-format; \
+  ln -s /usr/bin/clang-format-diff-13 /usr/bin/clang-format-diff; \
+  ln -s /usr/bin/git-clang-format-13 /usr/bin/git-clang-format; \
+  ln -s /usr/bin/lld-13 /usr/bin/lld; \
+  ln -s /usr/bin/lld-13 /usr/bin/ld.lld
+
+# Install clang-16, required for slang builds
+RUN wget https://apt.llvm.org/llvm.sh; \
+  chmod +x llvm.sh; \
+  ./llvm.sh 16;\
+  apt install -y clang-16
+
+# Install GCC 11 to get C++20 header support and support for building slang
+RUN add-apt-repository ppa:ubuntu-toolchain-r/test
+RUN apt install -y g++-11
 
 ADD https://github.com/Kitware/CMake/releases/download/v3.25.2/cmake-3.25.2-linux-x86_64.sh /tmp/cmake_install.sh
 RUN bash /tmp/cmake_install.sh --prefix=/usr/local --skip-license