Suppress warnings from Clang

Suppress warnings from Clang when compiling with Valgrind like this:

In file included from vm/capi/handle.cpp:3:
vm/gc/baker.hpp:274:7: error: expression result unused [-Werror,-Wunused-value]
      VALGRIND_MAKE_MEM_NOACCESS(next->start().as_int(), next->size());
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/valgrind/memcheck.h:110:5: note: expanded from:
    VALGRIND_DO_CLIENT_REQUEST_EXPR(0 /* default return */,      \
    ^
/usr/include/valgrind/valgrind.h:383:5: note: expanded from:
    _zzq_result;                                                  \
    ^~~~~~~~~~~

Please remove this when this bug (*1) is fixed and the fixed Valgrind is
available everywhere.

*1: https://bugs.kde.org/show_bug.cgi?id=316612