[libc++] Remove the legacy LIBCXX_LIBCPPABI_VERSION option

I think this notion of libc++abi's version was relevant a long time ago
on Apple platforms when we were using a Xcode project to build the library.
As part of moving Apple's build to CMake, D59489 made it possible to
specify the "ABI version" of libc++abi in use. However, it's not possible
to build libc++abi with that old ABI anymore and we don't need the ability
to link against that version from libc++ anymore.

Hence, we can clean this up and stop falsely pretending that libc++abi
has more than one ABI version.

Differential Revision: https://reviews.llvm.org/D125687

libcxx/CMakeLists.txt

@@ -202,8 +202,6 @@ option(LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT "Enable per TU ABI insulation by d
 set(LIBCXX_ABI_DEFINES "" CACHE STRING "A semicolon separated list of ABI macros to define in the site config header.")
 option(LIBCXX_EXTRA_SITE_DEFINES "Extra defines to add into __config_site")
 option(LIBCXX_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
-set(LIBCXX_LIBCPPABI_VERSION "2" CACHE STRING "Version of libc++abi's ABI to re-export from libc++ when re-exporting is enabled.
-                                               Note that this is not related to the version of libc++'s ABI itself!")
 
 # ABI Library options ---------------------------------------------------------
 if (LIBCXX_TARGETING_MSVC)

libcxx/src/CMakeLists.txt

@@ -251,7 +251,7 @@ if (LIBCXX_ENABLE_SHARED)
   if (LIBCXX_OSX_REEXPORT_LIBCXXABI_SYMBOLS)
     target_link_libraries(cxx_shared PRIVATE
       "-Wl,-unexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/libc++unexp.exp"
-      "-Wl,-reexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/libc++abi.v${LIBCXX_LIBCPPABI_VERSION}.exp"
+      "-Wl,-reexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/libc++abi.exp"
       "-Wl,-force_symbols_not_weak_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/notweak.exp"
       "-Wl,-force_symbols_weak_list,${CMAKE_CURRENT_SOURCE_DIR}/../lib/weak.exp")