CI: Add vanilla-LLVM 20 job

Author: kinke

.github/workflows/supported_llvm_versions.yml

@@ -19,6 +19,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
+          - job_name: Ubuntu 24.04, LLVM 20, latest LDC beta
+            os: ubuntu-24.04
+            host_dc: ldc-beta
+            llvm_version: 20
+            cmake_flags: -DCMAKE_EXE_LINKER_FLAGS=-lcurl  # work around libcurl dependency for ldc-profdata (LLVM apparently built with non-default LLVM_ENABLE_CURL=ON)
           - job_name: Ubuntu 24.04, LLVM 19, latest LDC beta
             os: ubuntu-24.04
             host_dc: ldc-beta