Add missing clang to the monolithic pre-merge build

Clang has a custom separate pipeline integrated with libc++
that only runs in release mode. It means that changes which touches
only clang won't run the clang tests in the configuration used by
LLVM premerge and will break it unknowingly.

Author: joker-eph

.ci/generate-buildkite-pipeline-premerge

@@ -224,7 +224,7 @@ fi
 # needs while letting them run on the infrastructure provided by LLVM.
 
 # Figure out which projects need to be built on each platform
-all_projects="bolt clang-tools-extra compiler-rt cross-project-tests flang libc libclc lld lldb llvm mlir openmp polly pstl"
+all_projects="bolt clang clang-tools-extra compiler-rt cross-project-tests flang libc libclc lld lldb llvm mlir openmp polly pstl"
 modified_projects="$(keep-modified-projects ${all_projects})"
 
 linux_projects_to_test=$(exclude-linux $(compute-projects-to-test ${modified_projects}))