HAVE_valgrind_or_MSAN to HAVE_valgrind was incorrect in
af784385.

In my_valgrind.h when clang exists (hence no __has_feature(memory_sanitizer),
and -DWITH_VALGRIND=1, but without memcheck.h, we end up with a MEM_CHECK_DEFINED
being empty.

If we are also doing a CMAKE_BUILD_TYPE=Debug this results a number of
[-Werror,-Wunused-variable] errors because MEM_CHECK_DEFINED is empty.
With MEM_CHECK_DEFINED empty, there becomes no uses of this of the
fixed field and innodb variables in this patch.

So we stop using HAVE_valgrind as catchall and use the name
HAVE_CHECK_MEM to indicate that a CHECK_MEM_DEFINED function exists.

Reviewer: Monty

Corrects: af784385