[libc++] Bump OS version for macOS backdeployment CI jobs

In 0547e57, I introduced backdeployment testing on macOS using
Github-provided builders. This was done by basically building libc++
on a slightly older macOS (like macOS 13) and then running against
the system library on that machine. However, that created a dependency
that libc++ must keep working on macOS 13, which doesn't support the
latest-released Xcode.

This patch solves that problem by moving the deployment testing to a
newer version of macOS which supports the latest-released version of
Xcode.

Sadly, that also reduces the backdeployment coverage we have since we're
not actually testing on older OSes, but is necessary to satisfy the
documented libc++ support policy. In the future, we could improve the
situation by providing a Lit configuration that allows compiling
(but not running) all the tests, building the tests on a supported macOS,
and then shipping those tests on an older backdeployment target in order
to run them against the system library. Since that requires significant
engineering, this isn't done at this time.

Author: ldionne

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

@@ -201,10 +201,20 @@ jobs:
           os: macos-15
         - config: apple-configuration
           os: macos-15
+        # TODO: These jobs are intended to test back-deployment (building against ToT libc++ but running against an
+        #       older system-provided libc++.dylib). Doing this properly would require building the test suite on a
+        #       recent macOS using a recent Clang (hence recent Xcode), and then running the actual test suite on an
+        #       older mac. We could do that by e.g. sharing artifacts between the two jobs.
+        #
+        #       However, our Lit configuration currently doesn't provide a good way to do that in a batch, so our only
+        #       alternative is to actually build on the same host that we're going to run on. Sadly, that doesn't work
+        #       since older macOSes don't support newer Xcodes. For now, we run the "backdeployment" jobs on recent
+        #       macOS versions as a way to avoid rotting that configuration, but it doesn't provide a lot of additional
+        #       coverage.
         - config: apple-system
-          os: macos-13
+          os: macos-15
         - config: apple-system-hardened
-          os: macos-13
+          os: macos-15
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2