[libc++] Bump OS version for macOS backdeployment CI jobs (#131883)

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.

(cherry picked from commit b4f7a2a)

Author: ldionne

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

@@ -197,10 +197,20 @@ 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@v4

libcxx/test/libcxx/strings/basic.string/string.capacity/allocation_size.pass.cpp

@@ -8,11 +8,6 @@
 
 // <string>
 
-// This test demonstrates the smaller allocation sizes when the alignment
-// requirements of std::string are dropped from 16 to 8.
-//
-// XFAIL: using-built-library-before-llvm-19
-
 #include <algorithm>
 #include <cassert>
 #include <cstddef>

libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/setbuf.pass.cpp

@@ -10,8 +10,9 @@
 
 // basic_streambuf<charT, traits>* setbuf(char_type* s, streamsize n) override;
 
-// In C++23 and later, this test requires support for P2467R1 in the dylib (a3f17ba3febbd546f2342ffc780ac93b694fdc8d)
-// XFAIL: (!c++03 && !c++11 && !c++14 && !c++17 && !c++20) && using-built-library-before-llvm-18
+// This test requires the fix to https://github.com/llvm/llvm-project/issues/60509 in the dylib,
+// which landed in 5afb937d8a30445642ccaf33866ee4cdd0713222.
+// XFAIL: using-built-library-before-llvm-19
 
 #include <fstream>
 #include <cstddef>

libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_double.pass.cpp

@@ -6,6 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+// The fix for LWG2381 (https://github.com/llvm/llvm-project/pull/77948) changed behavior of
+// FP parsing. This requires 3e15c97fa3812993bdc319827a5c6d867b765ae8 in the dylib.
+// XFAIL: using-built-library-before-llvm-19
+
 // <locale>
 
 // class num_get<charT, InputIterator>

libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_float.pass.cpp

@@ -6,6 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+// The fix for LWG2381 (https://github.com/llvm/llvm-project/pull/77948) changed behavior of
+// FP parsing. This requires 3e15c97fa3812993bdc319827a5c6d867b765ae8 in the dylib.
+// XFAIL: using-built-library-before-llvm-19
+
 // <locale>
 
 // class num_get<charT, InputIterator>

libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_long_double.pass.cpp

@@ -6,6 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+// The fix for LWG2381 (https://github.com/llvm/llvm-project/pull/77948) changed behavior of
+// FP parsing. This requires 3e15c97fa3812993bdc319827a5c6d867b765ae8 in the dylib.
+// XFAIL: using-built-library-before-llvm-19
+
 // <locale>
 
 // class num_get<charT, InputIterator>

libcxx/test/std/strings/basic.string/string.capacity/max_size.pass.cpp

@@ -8,12 +8,6 @@
 
 // UNSUPPORTED: no-exceptions
 
-// After changing the alignment of the allocated pointer from 16 to 8, the exception
-// thrown is no longer `bad_alloc` but instead length_error on systems using new
-// headers but a dylib that doesn't contain 04ce0ba.
-//
-// XFAIL: using-built-library-before-llvm-19
-
 // <string>
 
 // size_type max_size() const; // constexpr since C++20

libcxx/test/std/strings/basic.string/string.capacity/over_max_size.pass.cpp

@@ -8,6 +8,12 @@
 
 // UNSUPPORTED: no-exceptions
 
+// After changing the alignment of the allocated pointer from 16 to 8, the exception
+// thrown is no longer `bad_alloc` but instead length_error on systems using new
+// headers but a dylib that doesn't contain 04ce0ba.
+//
+// XFAIL: using-built-library-before-llvm-19
+
 // <string>
 
 // size_type max_size() const; // constexpr since C++20