[libc++] Perform the bootstrapping build before legacy builds in CI

This is to help reduce latency by running longer jobs before shorter ones.

Author: ldionne

libcxx/utils/ci/buildkite-pipeline.yml

@@ -241,8 +241,8 @@ steps:
     timeout_in_minutes: 120
 
   # Tests with the various supported ways to build libc++.
-  - label: "Legacy Lit configuration"
-    command: "libcxx/utils/ci/run-buildbot legacy-test-config"
+  - label: "Bootstrapping build"
+    command: "libcxx/utils/ci/run-buildbot bootstrapping-build"
     artifact_paths:
       - "**/test-results.xml"
     agents:
@@ -254,8 +254,8 @@ steps:
           limit: 2
     timeout_in_minutes: 120
 
-  - label: "Legacy standalone build"
-    command: "libcxx/utils/ci/run-buildbot legacy-standalone"
+  - label: "Legacy Lit configuration"
+    command: "libcxx/utils/ci/run-buildbot legacy-test-config"
     artifact_paths:
       - "**/test-results.xml"
     agents:
@@ -267,8 +267,8 @@ steps:
           limit: 2
     timeout_in_minutes: 120
 
-  - label: "Legacy LLVM_ENABLE_PROJECTS build"
-    command: "libcxx/utils/ci/run-buildbot legacy-project-build"
+  - label: "Legacy standalone build"
+    command: "libcxx/utils/ci/run-buildbot legacy-standalone"
     artifact_paths:
       - "**/test-results.xml"
     agents:
@@ -280,8 +280,8 @@ steps:
           limit: 2
     timeout_in_minutes: 120
 
-  - label: "Bootstrapping build"
-    command: "libcxx/utils/ci/run-buildbot bootstrapping-build"
+  - label: "Legacy LLVM_ENABLE_PROJECTS build"
+    command: "libcxx/utils/ci/run-buildbot legacy-project-build"
     artifact_paths:
       - "**/test-results.xml"
     agents: