[libc++] Fix extern C for __sanitizer_annotate_contiguous_container() (for gcc)

gcc supports it only at the beginning:

    $ g++ -o /dev/null -c /tmp/test_extern.cpp
    $ cat /tmp/test_extern.cpp
    extern "C" __attribute__ ((__visibility__("default"))) int foo();

Otherwise:

    $ g++ -o /dev/null -c /tmp/test_extern.cpp
    /tmp/test_extern.cpp:1:52: error: expected unqualified-id before string constant
        1 | __attribute__ ((__visibility__("default"))) extern "C" int foo();
          |                                                    ^~~
    $ cat /tmp/test_extern.cpp
    __attribute__ ((__visibility__("default"))) extern "C" int foo();

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D93316

Author: azat

libcxx/include/__config

@@ -1082,7 +1082,7 @@ typedef unsigned int   char32_t;
 #endif
 
 #ifndef _LIBCPP_HAS_NO_ASAN
-_LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
+extern "C" _LIBCPP_FUNC_VIS void __sanitizer_annotate_contiguous_container(
   const void *, const void *, const void *, const void *);
 #endif