[CI] Make premerge upload/write comments (#166609)

This only does this for Linux currently as the issue-write workflow
currently does not support writing out multiple comments. This gets the
ball rolling as the failures that most people see are common to both
platforms. Ensuring we have coverage on Windows for comments will be
done in a future patch.

Author: boomanaiden154

.ci/premerge_advisor_explain.py

@@ -39,6 +39,7 @@ def get_comment(
 ) -> dict[str, str]:
     repo = github.Github(github_token).get_repo("llvm/llvm-project")
     pr = repo.get_issue(pr_number).as_pull_request()
+    body = COMMENT_TAG.format(platform=platform.system()) + "\n" + body
     comment = {"body": body}
     comment_id = get_comment_id(platform.system(), pr)
     if comment_id:
@@ -128,7 +129,7 @@ def main(
                 ),
             )
         ]
-        with open("comment", "w") as comment_file_handle:
+        with open("comments", "w") as comment_file_handle:
             json.dump(comments, comment_file_handle)
     else:
         print(advisor_response.reason)

.github/workflows/premerge.yaml

@@ -119,6 +119,14 @@ jobs:
           path: artifacts/
           retention-days: 5
           include-hidden-files: 'true'
+      - name: Upload Comment
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
+        if: ${{ always() && !startsWith(matrix.runs-on, 'depot-ubuntu-24.04-arm') }}
+        continue-on-error: true
+        with:
+          name: workflow-args
+          path: |
+            comments
 
   premerge-checks-windows:
     name: Build and Test Windows