[Fuzzer] Set an explicit libc++ dependency when needed

[phosek]

Summary

On targets that don't link internal libc++ (Fuchsia and Linux) but
use libc++ as their C++ library and libFuzzer is being built using
the just built compiler together with libc++ as part of runtimes, we
need an explicit dependency from libFuzzer object library to libc++ to
make sure the headers are available by the time we start building
libFuzzer.

lib/fuzzer/CMakeLists.txt

CHECK_CXX_SOURCE_COMPILES("
   " HAS_THREAD_LOCAL)
 
 set(LIBFUZZER_CFLAGS ${SANITIZER_COMMON_CFLAGS})
 
 if(OS_NAME MATCHES "Linux|Fuchsia" AND COMPILER_RT_LIBCXX_PATH)
   list(APPEND LIBFUZZER_CFLAGS -nostdinc++ -D_LIBCPP_ABI_VERSION=Fuzzer)
   # Remove -stdlib= which is unused when passing -nostdinc++.
   string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+elseif(CLANG_DEFAULT_CXX_STDLIB STREQUAL "libc++" AND (TARGET cxx OR HAVE_LIBCXX))
+  set(LIBFUZZER_DEPS cxx)
 endif()
 
 append_list_if(COMPILER_RT_HAS_OMIT_FRAME_POINTER_FLAG -fno-omit-frame-pointer LIBFUZZER_CFLAGS)
 
 if (CMAKE_CXX_FLAGS MATCHES "fsanitize-coverage")
   list(APPEND LIBFUZZER_CFLAGS -fno-sanitize-coverage=trace-pc-guard,edge,trace-cmp,indirect-calls,8bit-counters)
 endif()
 
 if(NOT HAS_THREAD_LOCAL)
   list(APPEND LIBFUZZER_CFLAGS -Dthread_local=__thread)
 endif()
 
 set(FUZZER_SUPPORTED_OS ${SANITIZER_COMMON_SUPPORTED_OS})
 
 add_compiler_rt_object_libraries(RTfuzzer
   OS ${FUZZER_SUPPORTED_OS}
   ARCHS ${FUZZER_SUPPORTED_ARCH}
   SOURCES ${LIBFUZZER_SOURCES}
-  CFLAGS ${LIBFUZZER_CFLAGS})
+  CFLAGS ${LIBFUZZER_CFLAGS}
+  DEPS ${LIBFUZZER_DEPS})
 
 add_compiler_rt_object_libraries(RTfuzzer_main
   OS ${FUZZER_SUPPORTED_OS}
   ARCHS ${FUZZER_SUPPORTED_ARCH}
   SOURCES FuzzerMain.cpp
-  CFLAGS ${LIBFUZZER_CFLAGS})
+  CFLAGS ${LIBFUZZER_CFLAGS}
+  DEPS ${LIBFUZZER_DEPS})
 
 add_compiler_rt_runtime(clang_rt.fuzzer
   STATIC
   OS ${FUZZER_SUPPORTED_OS}
   ARCHS ${FUZZER_SUPPORTED_ARCH}
   OBJECT_LIBS RTfuzzer RTfuzzer_main
   CFLAGS ${LIBFUZZER_CFLAGS}
   PARENT_TARGET fuzzer)