[libc++] Shuffle the order of pre-commit CI jobs a bit (#128008)

I've recently noticed that our CI is bottlenecked around platforms on
which we don't have a lot of capacity like macOS (mostly) and Windows.
To try to alleviate that, this patch moves the macOS builds and the
Windows builds further down the pipeline so that they will get triggered
less often (if an earlier job fails).

Author: ldionne

.github/workflows/libcxx-build-and-test.yaml

@@ -124,7 +124,7 @@ jobs:
             **/crash_diagnostics/*
   stage3:
     if: github.repository_owner == 'llvm'
-    needs: [ stage1, stage2 ]
+    needs: [ stage2 ]
     continue-on-error: false
     strategy:
       fail-fast: false
@@ -188,7 +188,7 @@ jobs:
             **/crash_diagnostics/*
 
   macos:
-    needs: [ stage1 ]
+    needs: [ stage3 ]
     strategy:
       fail-fast: false
       matrix:
@@ -232,7 +232,7 @@ jobs:
 
   windows:
     runs-on: windows-2022
-    needs: [ stage1 ]
+    needs: [ stage2 ]
     strategy:
       fail-fast: false
       matrix: