[libc++] Force the ABI namespace to be a reserved identifier

[ldionne]

Summary

When the ABI namespace isn't a reserved identifier, we were issuing a
warning, but this should have been an error since the beginning. This
commit enforces that the ABI namespace is a reserved identifier, and
changes the ABI namespace used by LibFuzzer.

[phosek]

LGTM

compiler-rt/lib/fuzzer/CMakeLists.txt

if(OS_NAME MATCHES "Linux|Fuchsia" AND
 
   foreach(arch ${FUZZER_SUPPORTED_ARCH})
     get_target_flags_for_arch(${arch} TARGET_CFLAGS)
     set(LIBCXX_${arch}_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/libcxx_fuzzer_${arch})
     add_custom_libcxx(libcxx_fuzzer_${arch} ${LIBCXX_${arch}_PREFIX}
       CFLAGS ${TARGET_CFLAGS}
       CMAKE_ARGS -DCMAKE_CXX_COMPILER_WORKS=ON
                  -DCMAKE_POSITION_INDEPENDENT_CODE=ON
-                 -DLIBCXX_ABI_NAMESPACE=Fuzzer)
+                 -DLIBCXX_ABI_NAMESPACE=__Fuzzer)
     target_compile_options(RTfuzzer.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer.${arch} libcxx_fuzzer_${arch}-build)
     target_compile_options(RTfuzzer_main.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer_main.${arch} libcxx_fuzzer_${arch}-build)
     partially_link_libcxx(fuzzer_no_main ${LIBCXX_${arch}_PREFIX} ${arch})
     partially_link_libcxx(fuzzer ${LIBCXX_${arch}_PREFIX} ${arch})
   endforeach()
 endif()
 
 if(COMPILER_RT_INCLUDE_TESTS)
   add_subdirectory(tests)
 endif()

libcxx/CMakeLists.txt

 endfunction()
 
 # Configuration file flags =====================================================
 if (NOT LIBCXX_ABI_VERSION EQUAL 1)
   config_define(${LIBCXX_ABI_VERSION} _LIBCPP_ABI_VERSION)
 endif()
 if (NOT LIBCXX_ABI_NAMESPACE STREQUAL "")
   if (NOT LIBCXX_ABI_NAMESPACE MATCHES "__.*")
-    message(WARNING "LIBCXX_ABI_NAMESPACE must be a reserved identifier.")
+    message(FATAL_ERROR "LIBCXX_ABI_NAMESPACE must be a reserved identifier.")
   endif()
   if (LIBCXX_ABI_NAMESPACE MATCHES "__[0-9]+$")
     message(FATAL_ERROR "LIBCXX_ABI_NAMESPACE '${LIBCXX_ABI_NAMESPACE}' is reserved for use by libc++.")
   endif()
   config_define(${LIBCXX_ABI_NAMESPACE} _LIBCPP_ABI_NAMESPACE)
 endif()
 config_define_if(LIBCXX_ABI_UNSTABLE _LIBCPP_ABI_UNSTABLE)
 config_define_if(LIBCXX_ABI_FORCE_ITANIUM _LIBCPP_ABI_FORCE_ITANIUM)