[libcxx] Add "install-libcxx" target.

[EricWF]

Summary

Currently you can't install libc++ from within the LLVM tree without installing all of LLVM. This patch adds an install rule for libc++.

[EricWF]

D11682 got accepted and this patch is almost identical. Accepting this patch so it can be committed.

cmake/Modules/HandleLibCXXABI.cmake

           )
         install(FILES "${CMAKE_BINARY_DIR}/include/${fpath}"
           DESTINATION include/c++/v1/${dstdir}
+          COMPONENT libcxx
           PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
           )
         list(APPEND abilib_headers "${CMAKE_BINARY_DIR}/include/${fpath}")

include/CMakeLists.txt

 if (LIBCXX_INSTALL_HEADERS)
   install(DIRECTORY .
     DESTINATION include/c++/v1
+    COMPONENT libcxx
     FILES_MATCHING
     ${LIBCXX_HEADER_PATTERN}
     PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ

lib/CMakeLists.txt

   )
 
 install(TARGETS cxx
-  LIBRARY DESTINATION lib${LIBCXX_LIBDIR_SUFFIX}
-  ARCHIVE DESTINATION lib${LIBCXX_LIBDIR_SUFFIX}
+  LIBRARY DESTINATION lib${LIBCXX_LIBDIR_SUFFIX} COMPONENT libcxx
+  ARCHIVE DESTINATION lib${LIBCXX_LIBDIR_SUFFIX} COMPONENT libcxx
   )
+
+if (NOT CMAKE_CONFIGURATION_TYPES)
+  add_custom_target(install-libcxx
+                    DEPENDS cxx
+                    COMMAND "${CMAKE_COMMAND}"
+                    -DCMAKE_INSTALL_COMPONENT=libcxx
+                    -P "${LIBCXX_BINARY_DIR}/cmake_install.cmake")
+endif()