This adds preliminary support for handling the "br_on_exn" opcode in the
interpreter. It also makes "catch" and "rethrow" use a proper exception
reference instead of a dummy value.

To that end this also adds {Handle<>} as a new kind of {WasmValue} which
is intended to pass reference values (e.g. "anyref" or "except_ref") to
the runtime system. Therefore lifetime of such a {WasmValue} is directly
coupled to any surrounding {HandleScope}.

For now we just store {Handle<>} directly on the simulated operand stack
of the interpreter. This is of course bogus, since the surrounding scope
does not outlive the interpreter activation. Decoupling the lifetime of
the operand stack from a {HandleScope} will be done in a follow-up CL.

As a drive-by this change also implements support for the "ref_null" and
the "ref_is_null" opcodes as a proof-of-concept that the new {WasmValue}
is also applicable to the "anyref" reference type.

R=clemensh@chromium.org
TEST=cctest/test-run-wasm-interpreter/ReferenceTypeLocals
BUG=v8:8091,v8:7581

Change-Id: I2307e0689a19c4aab1d67f1ba6742cb3cc31aa3c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1550299
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60598}