Motivation: With rtt.sub now allowed in constant expressions, we have
to generalize WasmInitExpr to be able to handle expressions with
operands. This is the second CL that prepares the ground for this
change.

Changes:
- Remove the error from read-value-type when reading a generic rtt.
- Add validation for HeapTypeImmediate in ModuleDecoder. Use it to
  validate null constants immediates, which was missing. Add tests.
- Change ValueType::name to print rtt depths as integers.
- Move global initializer building in wasm-module-builder to its own
  function.

Bug: v8:7748
Change-Id: Ic041e1c7d032f2a1063a21fec1bfe96cb0d8120e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2284983
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68767}