Include omp{,-tools}.h in the releases

This is really ugly, but AFAICT:
 - omp.h is not a "builtin clang header" in the usual sense, but when
   openmp is built it installs to clang's resource dir to be "always
   available"
 - this makes it into clang packages, e.g. debian's libclang-common-11-dev.
 - as a result, if we don't ship it, then <omp.h> can be found by system
   clang but not by us.

.github/workflows/autobuild.yaml

@@ -263,7 +263,7 @@ jobs:
         cp llvm-project/llvm/LICENSE.TXT ${{ env.CLANGD_DIR }}
 
         cmake -G Ninja -S llvm-project/llvm -B ${{ env.CLANGD_DIR }}
-        "-DLLVM_ENABLE_PROJECTS=clang;clang-tools-extra"
+        "-DLLVM_ENABLE_PROJECTS=clang;clang-tools-extra;openmp"
         "-DLLVM_ENABLE_ASSERTIONS=OFF"
         "-DLLVM_ENABLE_BACKTRACES=ON"
         "-DLLVM_ENABLE_TERMINFO=OFF"
@@ -279,6 +279,9 @@ jobs:
       run: >
         ninja -C ${{ env.CLANGD_DIR }} clangd clangd-indexer clangd-index-server
         clangd-index-server-monitor
+    - name: Install OpenMP headers
+      run: >
+        cp ${{ env.CLANGD_DIR }}/projects/src/omp{,-tools}.h ${{ env.CLANGD_DIR }}/lib/clang/*/include
     - name: Archive clangd
       run: >
         7z a clangd.zip