Update LLVM+clang to 20.0 (pre-release)

Signed-off-by: Brian Cain <bcain@quicinc.com>

Dockerfile

@@ -62,16 +62,16 @@ ARG QEMU_REF=hexagon-sysemu-04-oct-2024
 ARG ARTIFACT_BASE
 ARG ARTIFACT_TAG
 
-ENV VER 19.1.5
+ENV VER 20.0.0-rc0
 ENV TOOLCHAIN_INSTALL /usr/local/clang+llvm-${VER}-cross-hexagon-unknown-linux-musl/
 ENV ROOT_INSTALL /usr/local/hexagon-unknown-linux-musl-rootfs
 ENV MAKE_TARBALLS 1
 #ENV HOST_LLVM_VERSION 10
 #ENV CMAKE_VER 3.16.6
 #ENV CMAKE_URL https://github.com/Kitware/CMake/releases/download/v3.16.6/cmake-3.16.6-Linux-x86_64.tar.gz
 
-ENV LLVM_SRC_URL https://github.com/llvm/llvm-project/archive/llvmorg-${VER}.tar.gz
-ENV LLVM_TESTS_SRC_URL https://github.com/llvm/llvm-test-suite/archive/llvmorg-${VER}.tar.gz
+ENV LLVM_SRC_URL https://github.com/llvm/llvm-project/archive/llvmorg-21-init.tar.gz
+ENV LLVM_TESTS_SRC_URL https://github.com/llvm/llvm-test-suite/archive/llvmorg-19.1.5.tar.gz
 ENV MUSL_SRC_URL https://github.com/quic/musl/archive/d125203fcb134febcde6ca32181554560b67c790.tar.gz
 ENV HEXMVM_SRC_URL https://github.com/quic/hexagonMVM/archive/v0.1.1.tar.gz
 ENV LINUX_SRC_URL https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.7.11.tar.xz