[CMake] Use just basename when copying C++ ABI headers

[phosek]

Summary

This avoids duplicate directories when the filename includes path.

This addresses PR39145

[mgorny]

Thanks. I can confirm that it fixes installing libsupc++ headers for us on the main ABI. Apparently the headers in 32-bit multilib build aren't installed but I see the same issue with libcxx-6.0.1, so it's not a regression (and probably a bug on our end).

[phosek]

Ping? Would it be possible to get an LGTM so I can land this?

[ldionne]

Do we have an open-source tester that would have caught that? Do we build against libsupc++ on Green Dragon?

cmake/Modules/HandleLibCXXABI.cmake

foreach(fpath ${LIBCXX_ABILIB_FILES})
     set(found FALSE)
     foreach(incpath ${LIBCXX_CXX_ABI_INCLUDE_PATHS})
       if (EXISTS "${incpath}/${fpath}")
         set(found TRUE)
         get_filename_component(dstdir ${fpath} PATH)
         get_filename_component(ifile ${fpath} NAME)
         set(src ${incpath}/${fpath})
 
-        set(dst ${LIBCXX_BINARY_INCLUDE_DIR}/${dstdir}/${fpath})
+        set(dst ${LIBCXX_BINARY_INCLUDE_DIR}/${dstdir}/${ifile})
         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 AND LIBCXX_HEADER_DIR)
           set(dst "${LIBCXX_HEADER_DIR}/include/c++/v1/${dstdir}/${fpath}")