[libc++] Remove Fuchsia-specific knowledge to pick the ABI version

[ldionne]

Summary

The ABI version used by libc++ is a configuration option just like any other
configuration option. It is a knob that can be used by vendors to customize
the libc++ that they ship. As such, we should not be hardcoding vendor-specific
configuration choices in libc++.

When building libc++ for Fuchsia, Fuchsia's build scripts should simply define
the libc++ ABI version to 2 -- this will result in the _LIBCPP_ABI_VERSION
macro being defined in the __config header that is generated when libc++ is
built and installed, which is the correct way to customize libc++'s behavior
for specific vendors.

[ldionne]

Clarify that version 2 of the ABI is _currently_ unstable, it is not _the_ unstable version.

[phosek]

LGTM

libcxx/CMakeLists.txt

 option(LIBCXX_INSTALL_SUPPORT_HEADERS "Install libc++ support headers." ON)
 cmake_dependent_option(LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY
         "Install libc++experimental.a" ON
         "LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY;LIBCXX_INSTALL_LIBRARY" OFF)
 cmake_dependent_option(LIBCXX_INSTALL_FILESYSTEM_LIBRARY
         "Install libc++fs.a" ON
         "LIBCXX_ENABLE_FILESYSTEM;LIBCXX_INSTALL_LIBRARY" OFF)
 
-if (FUCHSIA)
-  set(DEFAULT_ABI_VERSION 2)
-else()
-  set(DEFAULT_ABI_VERSION 1)
-endif()
-set(LIBCXX_ABI_VERSION ${DEFAULT_ABI_VERSION} CACHE STRING "ABI version of libc++.")
+set(LIBCXX_ABI_VERSION "1" CACHE STRING "ABI version of libc++. Can be either 1 or 2, where 2 is the unstable version of the ABI. Defaults to 1.")
 option(LIBCXX_ABI_UNSTABLE "Unstable ABI of libc++." OFF)
 option(LIBCXX_ABI_FORCE_ITANIUM "Ignore auto-detection and force use of the Itanium ABI.")
 option(LIBCXX_ABI_FORCE_MICROSOFT "Ignore auto-detection and force use of the Microsoft ABI.")
 option(LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT "Enable per TU ABI insulation by default. To be used by vendors." OFF)
 set(LIBCXX_ABI_DEFINES "" CACHE STRING "A semicolon separated list of ABI macros to define in the site config header.")
 option(LIBCXX_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 
 if (NOT LIBCXX_ENABLE_SHARED AND NOT LIBCXX_ENABLE_STATIC)
else()
       message(WARNING "Unsupported value of LLVM_USE_SANITIZER: ${LLVM_USE_SANITIZER}")
     endif()
   elseif(LLVM_USE_SANITIZER AND MSVC)
     message(WARNING "LLVM_USE_SANITIZER is not supported on this platform.")
   endif()
 endif()
 
 # Configuration file flags =====================================================
-if (NOT LIBCXX_ABI_VERSION EQUAL DEFAULT_ABI_VERSION)
+if (NOT LIBCXX_ABI_VERSION EQUAL 1)
   config_define(${LIBCXX_ABI_VERSION} _LIBCPP_ABI_VERSION)
 endif()
 config_define_if(LIBCXX_ABI_UNSTABLE _LIBCPP_ABI_UNSTABLE)
 config_define_if(LIBCXX_ABI_FORCE_ITANIUM _LIBCPP_ABI_FORCE_ITANIUM)
 config_define_if(LIBCXX_ABI_FORCE_MICROSOFT _LIBCPP_ABI_FORCE_MICROSOFT)
 config_define_if(LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT _LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT)
 
 config_define_if_not(LIBCXX_ENABLE_GLOBAL_FILESYSTEM_NAMESPACE _LIBCPP_HAS_NO_GLOBAL_FILESYSTEM_NAMESPACE)

libcxx/include/__config

 #else
 #  define _GNUC_VER 0
 #  define _GNUC_VER_NEW 0
 #endif
 
 #define _LIBCPP_VERSION 8000
 
 #ifndef _LIBCPP_ABI_VERSION
-#  ifdef __Fuchsia__
-#    define _LIBCPP_ABI_VERSION 2
-#  else
-#    define _LIBCPP_ABI_VERSION 1
-#  endif
-#endif
+#  define _LIBCPP_ABI_VERSION 1
+#endif
 
 #ifndef _LIBCPP_STD_VER
 #  if  __cplusplus <= 201103L
 #    define _LIBCPP_STD_VER 11
 #  elif __cplusplus <= 201402L
 #    define _LIBCPP_STD_VER 14
 #  elif __cplusplus <= 201703L
 #    define _LIBCPP_STD_VER 17