[ci] test against clang 11

.gitlab-ci.yml

@@ -27,5 +27,11 @@ tests-on-llvm-10:
   stage: tests
   script:
    - nix/frama-ci.sh build -A frama-clang.on-llvm10.tests
+  when: manual
+
+tests-on-llvm-11:
+  stage: tests
+  script:
+   - nix/frama-ci.sh build -A frama-clang.on-llvm11.tests
   tags:
    - nix

nix/default.nix

@@ -30,3 +30,4 @@ let frama_clang_build =
 in
 (frama_clang_build { llvm_version="9"; })
 // { on-llvm10 = (frama_clang_build { llvm_version="10"; });}
+// { on-llvm11 = (frama_clang_build { llvm_version="11"; });}