[libc++][CI] Pin the XCode version. (llvm#135412)

This version is the current version, to avoid unplanned automatic
updates in the future pin this version.

Author: mordante

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

@@ -216,7 +216,8 @@ jobs:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - uses: maxim-lobanov/setup-xcode@60606e260d2fc5762a71e64e74b2174e8ea3c8bd # v1.6.0
         with:
-          xcode-version: 'latest'
+          # https://github.com/actions/runner-images/blob/main/images/macos/macos-15-Readme.md
+          xcode-version: '16.3'
       - uses: seanmiddleditch/gha-setup-ninja@3b1f8f94a2f8254bd26914c4ab9474d4f0015f67 # v6
       - name: Build and test
         run: |