Add libcxxabi option back for sanitizer use.

[srhines]

Summary

A prior refactoring accidentally dropped the case for using libc++abi as
the out-of-tree C++ runtime library for sanitizers. This patch restores
that functionality, which is used by Android, which can't depend on the
full libc++ for these libraries.

[srhines]

https://reviews.llvm.org/D47100 is the commit where this got dropped.

[phosek]

LGTM

CMakeLists.txt

356

Should the name be libc++abi for consistency with libc++ above?

[srhines]

Thanks for the quick review.

CMakeLists.txt

356

The original naming was libcxxabi, so I am restoring that (in case anyone else is also using it).

CMakeLists.txt

if (SANITIZER_CXX_ABI_INTREE)
     if (NOT LIBCXX_ENABLE_STATIC_ABI_LIBRARY AND (TARGET cxxabi_shared OR HAVE_LIBCXXABI))
       list(APPEND SANITIZER_CXX_ABI_LIBRARY cxxabi_shared)
     elseif (LIBCXX_ENABLE_STATIC_ABI_LIBRARY AND (TARGET cxxabi_static OR HAVE_LIBCXXABI))
       list(APPEND SANITIZER_CXX_ABI_LIBRARY cxxabi_static)
     endif()
   else()
     append_list_if(COMPILER_RT_HAS_LIBCXX c++ SANITIZER_CXX_ABI_LIBRARY)
   endif()
+elseif (SANITIZER_CXX_ABI_LIBNAME STREQUAL "libcxxabi")
+  list(APPEND SANITIZER_CXX_ABI_LIBRARY "c++abi")
 elseif (SANITIZER_CXX_ABI_LIBNAME STREQUAL "libstdc++")
   append_list_if(COMPILER_RT_HAS_LIBSTDCXX stdc++ SANITIZER_CXX_ABI_LIBRARY)
 endif()
 
 # Warnings to turn off for all libraries, not just sanitizers.
 append_string_if(COMPILER_RT_HAS_WUNUSED_PARAMETER_FLAG -Wno-unused-parameter CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
 if (CMAKE_LINKER MATCHES "link.exe$")