Add missing clang to the monolithic pre-merge build (llvm#85354)

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

@@ -68,7 +68,7 @@ function compute-projects-to-test() {
       done
     ;;
     clang)
-      for p in clang-tools-extra compiler-rt flang libc lldb openmp cross-project-tests; do
+      for p in clang-tools-extra compiler-rt flang lldb cross-project-tests; do
         echo $p
       done
     ;;
@@ -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}))