[CMake] Use libcxx targets for in-tree sanitizer C++ ABI

[phosek]

Summary

When in-tree libcxx is selected as the sanitizer C++ ABI, use libcxx
targets rather than libcxxabi and libunwind, since those may not be
available on all platforms (Windows for example).

[paulkirth]

LGTM modulo two small questions.

compiler-rt/CMakeLists.txt

582–585

Should we document this directly here?

583–587

If the unwinder bits here no longer apply, then do we still need the TODO above?

[phosek]

compiler-rt/CMakeLists.txt

582–585

I don't think so, the use of libcxxabi and libunwind instead of libcxx is for legacy reasons that are no longer relevant. Using libcxx targets when you set SANITIZER_CXX_ABI_LIBNAME=libc++ shouldn't be surprising, adding the comment about libcxxabi and libunwind here would just add confusion.

583–587

This TODO is still relevant, although it's primarily useful to me as a future reminder when I start refactoring this code further.

[phosek]

Rework the patch to make sure we use the correct unwinder and C++ ABI.

[hans]

This caused our builds to break:

CMake Error at /b/s/w/ir/cache/builder/src/third_party/llvm/compiler-rt/cmake/Modules/AddCompilerRT.cmake:383 (target_link_libraries):
  Error evaluating generator expression:

    $<TARGET_LINKER_FILE:$<IF:$<TARGET_EXISTS:libcxx-abi-shared>,libcxx-abi-shared,libcxx-abi-static>>

  No target "libcxx-abi-static"
Call Stack (most recent call first):
  /b/s/w/ir/cache/builder/src/third_party/llvm/compiler-rt/lib/ubsan/CMakeLists.txt:242 (add_compiler_rt_runtime)

see https://bugs.chromium.org/p/chromium/issues/detail?id=1370500#c1 for a reproducer.

I've reverted in 20a269cf774e774fb5c7194b1aebe24df27a233f until this is figured out.

compiler-rt/CMakeLists.txt

 endif()
 
 if (SANITIZER_CXX_ABI_LIBNAME STREQUAL "libc++")
   if (SANITIZER_CXX_ABI_INTREE)
     # TODO: We don't need to add --unwindlib=none to SANITIZER_COMMON_LINK_FLAGS
     # because we added -nodefaultlibs there earlier, and adding would result in
     # a warning, but if we switch to -nostdlib++, we would need to add it here.
     # append_list_if(CXX_SUPPORTS_UNWINDLIB_NONE_FLAG --unwindlib=none SANITIZER_COMMON_LINK_FLAGS)
-
-    if (SANITIZER_USE_STATIC_LLVM_UNWINDER AND (TARGET unwind_static OR HAVE_LIBUNWIND))
-      list(APPEND SANITIZER_CXX_ABI_LIBRARIES unwind_static)
-    elseif (TARGET unwind_shared OR HAVE_LIBUNWIND)
-      list(APPEND SANITIZER_CXX_ABI_LIBRARIES unwind_shared)
-    endif()
-
-    if (SANITIZER_USE_STATIC_CXX_ABI AND (TARGET cxxabi_static OR HAVE_LIBCXXABI))
-      list(APPEND SANITIZER_CXX_ABI_LIBRARIES cxxabi_static)
-    elseif (TARGET cxxabi_shared OR HAVE_LIBCXXABI)
-      list(APPEND SANITIZER_CXX_ABI_LIBRARIES cxxabi_shared)
+    if(SANITIZER_USE_STATIC_CXX_ABI)
+      set(SANITIZER_CXX_ABI_LIBRARIES "$<TARGET_LINKER_FILE:cxx_static>")
+    else()
+      set(SANITIZER_CXX_ABI_LIBRARIES "$<TARGET_LINKER_FILE:$<IF:$<TARGET_EXISTS:cxx_shared>,cxx_shared,cxx_static>>")
     endif()
   else()
     append_list_if(COMPILER_RT_HAS_LIBCXX c++ SANITIZER_CXX_ABI_LIBRARIES)
   endif()
 elseif (SANITIZER_CXX_ABI_LIBNAME STREQUAL "libcxxabi")
   list(APPEND SANITIZER_CXX_ABI_LIBRARIES "c++abi")
 elseif (SANITIZER_CXX_ABI_LIBNAME STREQUAL "libstdc++")
   append_list_if(COMPILER_RT_HAS_LIBSTDCXX stdc++ SANITIZER_CXX_ABI_LIBRARIES)