[llvm] Point workflow yaml at new docker image (PR #133359)

[George Burgess IV via llvm-commits]

https://github.com/gburgessiv updated https://github.com/llvm/llvm-project/pull/133359

>From 2332c9703a5682bc33c3e3112167b226c8f046b5 Mon Sep 17 00:00:00 2001
From: George Burgess IV <george.burgess.iv at gmail.com>
Date: Thu, 27 Mar 2025 20:48:47 -0600
Subject: [PATCH] .github: rely on new llvm-project checkout

This is now baked into the docker images to speed up
premerge cycle times.
https://github.com/llvm/llvm-project/pull/133359#issuecomment-2847434034
has initial numbers on the benefits we see in practice on bots.
---
 .github/workflows/premerge.yaml | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/.github/workflows/premerge.yaml b/.github/workflows/premerge.yaml
index c315ee37646d2..c68e384024cc5 100644
--- a/.github/workflows/premerge.yaml
+++ b/.github/workflows/premerge.yaml
@@ -31,10 +31,21 @@ jobs:
         (github.event_name != 'pull_request' || github.event.action != 'closed')
     runs-on: llvm-premerge-linux-runners
     steps:
+      - name: Seed LLVM repo
+        run: |
+          # Github creates a directory at GITHUB_WORKSPACE by defualt; replace it
+          # with a symlink to our pre-checked-out LLVM repo (this is necessary
+          # since actions/checkout _really wants_ a path inside of
+          # `${GITHUB_WORKSPACE}`
+          rmdir "${GITHUB_WORKSPACE}"
+          ln -s /home/gha/llvm-project "${GITHUB_WORKSPACE}"
       - name: Checkout LLVM
         uses: actions/checkout at 11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           fetch-depth: 2
+          # The `ln`'ed repo is frozen with the docker container; no need to
+          # clean.
+          clean: false
       - name: Setup ccache
         uses: hendrikmuhs/ccache-action at a1209f81afb8c005c13b4296c32e363431bffea5 # v1.2.17
         with: