[OCaml] Do not use -warn-error in tests.

[OCaml] Do not use -warn-error in tests.

This -warn-error flag invariably gets into release tarballs
and breaks builds on distributions that run tests as a part
of release process. The OCaml binding tests are especially
critical, since they often expose lingering toolchain bugs,
and so it is replaced with -w +A (equivalent to -Wall).

llvm-svn: 242550