[libc++][libc++abi] Add tests for vendor-specific properties

Vendors take libc++ and ship it in various ways. Some vendors might
ship it differently from what upstream LLVM does, i.e. the install
location might be different, some ABI properties might differ, etc.

In the past few years, I've come across several instances where
having a place to test some of these properties would have been
incredibly useful. I also just got bitten by the lack of tests
of that kind, so I'm adding some now.

The tests added by this commit for Apple platforms have numerous
TODOs that capture discrepancies between the upstream LLVM CMake
and the slightly-modified build we perform internally to produce
Apple's system libc++. In the future, the goal would be to upstream
all those differences so that it's possible to build a faithful
Apple system libc++ with the upstream LLVM sources only.

But this isn't only useful for Apple - this lays out the path for
any vendor being able to add their own checks (either upstream or
downstream) to libc++.

Differential Revision: https://reviews.llvm.org/D110736

GitOrigin-RevId: 9892d1644f62cf8f5133ee0d2ebafb56f6500cc1
diff --git a/test/lit.site.cfg.in b/test/lit.site.cfg.in
index 049f822..585e03e 100644
--- a/test/lit.site.cfg.in
+++ b/test/lit.site.cfg.in
@@ -7,6 +7,7 @@
 
 config.cxx_under_test           = "@CMAKE_CXX_COMPILER@"
 config.project_obj_root         = "@CMAKE_BINARY_DIR@"
+config.install_root             = "@CMAKE_BINARY_DIR@"
 config.libcxxabi_hdr_root       = "@LIBCXXABI_HEADER_DIR@"
 config.libcxxabi_src_root       = "@LIBCXXABI_SOURCE_DIR@"
 config.libcxxabi_obj_root       = "@LIBCXXABI_BINARY_DIR@"
diff --git a/test/vendor/apple/system-install-properties.sh.cpp b/test/vendor/apple/system-install-properties.sh.cpp
new file mode 100644
index 0000000..791fc67
--- /dev/null
+++ b/test/vendor/apple/system-install-properties.sh.cpp
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: stdlib=apple-libc++
+
+// This file checks various properties of the installation of libc++abi when built
+// as a system library on Apple platforms.
+//
+// TODO: We should install to `<prefix>/usr` in CMake and check that path instead.
+
+// Make sure we install the libc++abi headers in the right location.
+// TODO: We don't currently install them, but we should.
+//
+// XRUNX: stat "%{install}/include/cxxabi.h"
+
+// Make sure we install libc++abi.dylib in the right location.
+//
+// RUN: stat "%{install}/lib/libc++abi.dylib"
+
+// Make sure we don't install a symlink from libc++abi.dylib to libc++abi.1.dylib,
+// unlike what we do for libc++.dylib.
+// TODO: We currently don't do that correctly in the CMake build.
+//
+// XRUNX: ! readlink "%{install}/lib/libc++abi.dylib"
+// XRUNX: ! stat "%{install}/lib/libc++abi.1.dylib"
+
+// Make sure the install_name is /usr/lib.
+//
+// In particular, make sure we don't use any @rpath in the load commands. When building as
+// a system library, it is important to hardcode the installation paths in the dylib, because
+// various tools like dyld and ld64 will treat us specially if they recognize us as being a
+// system library.
+//
+// TODO: We currently don't do that correctly in the CMake build.
+//
+// XRUNX: otool -L "%{install}/lib/libc++abi.dylib" | grep '/usr/lib/libc++abi.dylib'
+// XRUNX: ! otool -l "%{install}/lib/libc++abi.dylib" | grep -E "LC_RPATH|@loader_path|@rpath"
+
+// Make sure the compatibility_version of libc++abi is 1.0.0. Failure to respect this can result
+// in applications not being able to find libc++abi when they are loaded by dyld, if the
+// compatibility version was bumped.
+//
+// RUN: otool -L "%{install}/lib/libc++abi.dylib" | grep "libc++abi.1.dylib" | grep "compatibility version 1.0.0"