[libc++] Take 2: 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
because it broke the build bots. However, the build bots now specify
explicitly what ABI library they want to use (libc++abi), so this
commit should now be OK to merge.

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365326 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -149,22 +149,9 @@ set_property(CACHE LIBCXX_CXX_ABI PROPERTY STRINGS ;${CXXABIS})
 
 # 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)