[libc++] Take 3: Do not cleverly link against libc++abi just because it happens to be there

Summary:
Otherwise, when libcxxabi is not an enabled project in the monorepo, we
get a link error because we try to link against non-existent cxxabi_shared.

More generally, we shouldn't change the behavior of the build based on
implicit things like whether a file happens to be at a specific path or
not.

This is a re-application of r365222 that had been reverted in r365233
and then r365359 because it broke the build bots. The build bots
should now specify explicitly what ABI library they want to use
(libc++abi), so this commit should now be OK to merge. It takes a while
for build bots to pick up configuration changes, which is why this failed
the last time around.

Differential Revision: https://reviews.llvm.org/D63883

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@368213 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -160,22 +160,9 @@ if (DEFINED ENV{USER})
 endif()
 # Setup the default options if LIBCXX_CXX_ABI is not specified.
 if (LIBCXX_CXX_ABI STREQUAL "default")
-  find_path(
-    LIBCXX_LIBCXXABI_INCLUDES_INTERNAL cxxabi.h
-    PATHS ${LLVM_MAIN_SRC_DIR}/projects/libcxxabi/include
-          ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi/include
-          ${LLVM_MAIN_SRC_DIR}/../libcxxabi/include
-    NO_DEFAULT_PATH
-    NO_CMAKE_FIND_ROOT_PATH
-  )
   if (LIBCXX_TARGETING_MSVC)
     # FIXME: Figure out how to configure the ABI library on Windows.
     set(LIBCXX_CXX_ABI_LIBNAME "vcruntime")
-  elseif ((NOT LIBCXX_STANDALONE_BUILD OR HAVE_LIBCXXABI) AND
-          IS_DIRECTORY "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
-    set(LIBCXX_CXX_ABI_LIBNAME "libcxxabi")
-    set(LIBCXX_CXX_ABI_INCLUDE_PATHS "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
-    set(LIBCXX_CXX_ABI_INTREE 1)
   elseif (APPLE)
     set(LIBCXX_CXX_ABI_LIBNAME "libcxxabi")
     set(LIBCXX_CXX_ABI_SYSTEM 1)

cmake/Modules/HandleLibCXXABI.cmake

@@ -98,7 +98,7 @@ if ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libstdc++" OR
     "${_LIBSUPCXX_LIBNAME}" "${_LIBSUPCXX_LIBNAME}" "${_LIBSUPCXX_INCLUDE_FILES}" "bits"
     )
 elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libcxxabi")
-  if (LIBCXX_CXX_ABI_INTREE)
+  if (TARGET cxxabi_shared AND TARGET cxxabi_static)
     # Link against just-built "cxxabi" target.
     set(CXXABI_SHARED_LIBNAME cxxabi_shared)
     set(CXXABI_STATIC_LIBNAME cxxabi_static)