[libcxx] Move to using libc++abi2.exp as the default symbol list for libc++

[EricWF]

Summary

libc++abi2.exp should be used whenever cxxabi.h defines _LIBCPPABI_VERSION. This macro was added to libc++abi in 2012 in r149632. For this reason we should use libc++abi2.exp as default unless otherwise specified.

Also when building against an in-tree libc++abi we definitely want to use libc++abi2.exp.

I would love to know what OSX was the last to use libc++abi.exp but I can only test on 10.9.

[EricWF]

Don't manually link in libc++abi during testing on OS X because the clang linker driver doesn't. The current behavior was hiding a bunch of test failures.

[mclow.lists]

I'm ok with this.

[meadori]

LGTM. I also verified the problem I reported is fixed now.

test/libcxx/test/config.py

446 ↗

(On Diff #20356)

Minor nit, but you can inline the "target_platform" since it is not used anywhere else:

if self.target_info.platform() != 'darwin':

[EricWF]

Update and address comments.

[EricWF]

Accepting.

cmake/Modules/HandleLibCXXABI.cmake

   if (LIBCXX_CXX_ABI_INTREE)
     # Link against just-built "cxxabi" target.
     set(CXXABI_LIBNAME cxxabi)
+    set(LIBCXX_LIBCPPABI_VERSION "2" PARENT_SCOPE)
   else()
     # Assume c++abi is installed in the system, rely on -lc++abi link flag.
     set(CXXABI_LIBNAME "c++abi")

lib/CMakeLists.txt

 append_if(LIBCXX_LINK_FLAGS LIBCXX_HAS_NODEFAULTLIBS_FLAG -nodefaultlibs)
 
 if ( APPLE )
+  if (NOT DEFINED LIBCXX_LIBCPPABI_VERSION)
+    set(LIBCXX_LIBCPPABI_VERSION "2")
+  endif()
+
   if ( CMAKE_OSX_DEPLOYMENT_TARGET STREQUAL "10.6" )
     list(APPEND LIBCXX_COMPILE_FLAGS "-U__STRICT_ANSI__")
     list(APPEND LIBCXX_LINK_FLAGS