[libc++] Introduce the notion of a minimum header version (#166074)

Introducing the notion of a minimum header version has multiple
benefits. It allows us to merge a bunch of ABI macros into a single one.
This makes configuring the library significantly easier, since, for a
stable ABI, you only need to know which version you started distributing
the library with, instead of checking which ABI flags have been
introduced at what point. For platforms which have a moving window of
the minimum version a program has been compiled against, this also makes
it simple to remove symbols from the dylib when they can't be used by
any program anymore.

NOKEYCHECK=True
GitOrigin-RevId: 484ee42096f8deeb084c8b36548d09c12b33cde0

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3dabd87..2b67c59 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -64,6 +64,9 @@
 option(LIBCXXABI_HAS_EXTERNAL_THREAD_API
   "Build libc++abi with an externalized threading API.
   This option may only be set to ON when LIBCXXABI_ENABLE_THREADS=ON." OFF)
+set(LIBCXXABI_AVAILABILITY_MINIMUM_HEADER_VERSION "2" CACHE STRING
+  "Minimum version of the libc++ headers that are allowed to be used by binaries linked against the libc++ library. This
+   version must correspond to the major version of an LLVM release.")
 option(LIBCXXABI_ENABLE_FORGIVING_DYNAMIC_CAST
 "Make dynamic_cast more forgiving when type_info's mistakenly have hidden \
 visibility, and thus multiple type_infos can exist for a single type. \
@@ -321,6 +324,7 @@
 # libcxxabi needs to, for various reasons, include the libcpp headers as if
 # it is being built as part of libcxx.
 add_definitions(-D_LIBCPP_BUILDING_LIBRARY)
+add_definitions(-D_LIBCPP_AVAILABILITY_MINIMUM_HEADER_VERSION=${LIBCXXABI_AVAILABILITY_MINIMUM_HEADER_VERSION})
 
 # Get feature flags.
 add_compile_flags_if_supported(-fstrict-aliasing)