[Github] Add steps to build clang docs to CI (#69550)

Recently, support for building the LLVM documentation within Github
actions landed, allowing for easy testing of the docs both pre and post
landing. This patch extends that functionality to clang and adds in
additional support to the docs Github workflow to only build the docs
for the subproject whose documentation has been touched.

Author: boomanaiden154

.github/workflows/docs.yml

@@ -14,9 +14,11 @@ on:
       - 'main'
     paths:
       - 'llvm/docs/**'
+      - 'clang/docs/**'
   pull_request:
     paths:
       - 'llvm/docs/**'
+      - 'clang/docs/**'
 
 jobs:
   check-docs-build:
@@ -39,10 +41,23 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get install -y cmake ninja-build
-      - name: Build docs
+      - name: Get subprojects that have doc changes
+        id: docs-changed-subprojects
+        uses: tj-actions/changed-files@v39
+        with:
+          files_yaml: |
+            llvm:
+              - 'llvm/docs/**'
+            clang:
+              - 'clang/docs/**'
+      - name: Build LLVM docs
+        if: steps.docs-changed-subprojects.outputs.llvm_any_changed == 'true'
+        run: |
+          cmake -B llvm-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_SPHINX=ON -DSPHINX_OUTPUT_HTML=ON -DSPHINX_OUTPUT_MAN=ON ./llvm
+          TZ=UTC ninja -C llvm-build docs-llvm-html docs-llvm-man
+      - name: Build Clang docs
+        if: steps.docs-changed-subprojects.outputs.clang_any_changed == 'true'
         run: |
-          mkdir build
-          cd build
-          cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_SPHINX=ON -DSPHINX_OUTPUT_HTML=ON -DSPHINX_OUTPUT_MAN=ON ../llvm
-          TZ=UTC ninja docs-llvm-html docs-llvm-man
+          cmake -B clang-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_SPHINX=ON -DSPHINX_OUTPUT_HTML=ON -DSPHINX_OUTPUT_MAN=ON ./llvm
+          TZ=UTC ninja -C clang-build docs-clang-html docs-clang-man