[CMake] Don't install c++abi headers in standalone libc++ build

[phosek]

Summary

This is a refinement on r337833. Previously we were installing two
copies of c++abi headers in libc++ build directory, one in
include/c++build and another one in include/c++/v1. However, the
second copy is unnecessary when building libc++ standalone.

[phosek]

This was pointed out by @mclow.lists on IRC in our discussion related to D49711, I believe this change should resolve that issue.

[mclow.lists]

With this change, only one copy of the c++abi headers gets copied - into include/c++/v1/.
LGTM.

libcxx/CMakeLists.txt

if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
   set(DEFAULT_INSTALL_HEADER_PREFIX lib${LLVM_LIBDIR_SUFFIX}/clang/${CLANG_VERSION}/)
   set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/clang/${CLANG_VERSION}/${LLVM_DEFAULT_TARGET_TRIPLE}/lib${LIBCXX_LIBDIR_SUFFIX})
   set(LIBCXX_HEADER_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/clang/${CLANG_VERSION})
 elseif(LLVM_LIBRARY_OUTPUT_INTDIR)
   set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
   set(LIBCXX_HEADER_DIR  ${LLVM_BINARY_DIR})
 else()
   set(LIBCXX_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})
-  set(LIBCXX_HEADER_DIR  ${CMAKE_BINARY_DIR})
 endif()
 
 file(MAKE_DIRECTORY "${LIBCXX_BINARY_INCLUDE_DIR}")
 
 set(LIBCXX_INSTALL_PREFIX ${DEFAULT_INSTALL_PREFIX} CACHE STRING
     "Define libc++ destination prefix.")
 
 set(LIBCXX_INSTALL_HEADER_PREFIX ${DEFAULT_INSTALL_HEADER_PREFIX} CACHE STRING

libcxx/cmake/Modules/HandleLibCXXABI.cmake

foreach(incpath ${LIBCXX_CXX_ABI_INCLUDE_PATHS})
 
         set(dst ${LIBCXX_BINARY_INCLUDE_DIR}/${dstdir}/${fpath})
         add_custom_command(OUTPUT ${dst}
             DEPENDS ${src}
             COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
             COMMENT "Copying C++ ABI header ${fpath}...")
         list(APPEND abilib_headers "${dst}")
 
-        if (NOT LIBCXX_USING_INSTALLED_LLVM)
+        if (NOT LIBCXX_USING_INSTALLED_LLVM AND LIBCXX_HEADER_DIR)
           set(dst "${LIBCXX_HEADER_DIR}/include/c++/v1/${dstdir}/${fpath}")
           add_custom_command(OUTPUT ${dst}
               DEPENDS ${src}
               COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
               COMMENT "Copying C++ ABI header ${fpath}...")
           list(APPEND abilib_headers "${dst}")
         endif()
 

libcxx/include/CMakeLists.txt

if (LIBCXX_NEEDS_SITE_CONFIG)
   set(generated_config_deps cxx-generated-config)
 else()
   set(files
     ${files}
     __config
     )
 endif()
 
-if(NOT LIBCXX_USING_INSTALLED_LLVM AND LLVM_BINARY_DIR)
+if(NOT LIBCXX_USING_INSTALLED_LLVM AND LIBCXX_HEADER_DIR)
   set(output_dir ${LIBCXX_HEADER_DIR}/include/c++/v1)
 
   set(out_files)
   foreach(f ${files})
     set(src ${CMAKE_CURRENT_SOURCE_DIR}/${f})
     set(dst ${output_dir}/${f})
     add_custom_command(OUTPUT ${dst}
       DEPENDS ${src}