With this CL we support importing mutable anyref globals. The CL
contains the following changes:

* We store a reference to the buffer of the imported global in
  `WasmInstance::imported_mutable_globals_buffer`. This FixedArray is
  already used to keep the ArrayBuffers of value-typed imported
  mutable globals alive but was not accessed otherwise.

* We store the offset in the buffer of the imported global in
  `WasmInstance::imported_mutable_globals`. This `Address`-array is
  used for value-typed imported mutalbe globals to store direct
  pointers into the backing store of the ArrayBuffer of the imported
  global.

* In wasm-compiler.cc we generate code to load these fields and then
  load or store globals.

* in module-compiler.cc I removed the counter variable
  `next_imported_mutable_global_index`. The variable was only used for
  a DCHECK. I replaced the DCHECK with a slightly weaker DCHECK now.

* Tests.

R=mstarzinger@chromium.org

Bug: v8:7581
Change-Id: I11f0f4d8637c98eded5fb2eec44bc8ead8ed5c7b
Reviewed-on: https://chromium-review.googlesource.com/c/1409169
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58822}