[tint][msl] ArgBufs: emit `_ =` statements unambiguously

Without argbufs, a `_ =` of a uniform/storage value results in:

  uint3((*tint_module_vars.f));

However, with argbufs, `f` becomes a function argument resulting in:

  uint3((*f));

  program_source:30:10: error: pointer type must have explicit address
  space qualifier

This is supposed to be an `expression;` but in C++ it gets parsed
incorrectly as a variable declaration, like `uint3* f;`.
Add parentheses around the expression so it can't be parsed that way:

  (uint3((*f)));

Note, this doesn't happen with bare constructor calls (see call.wgsl)
because in that case phony assignments get emitted for example as:

  int3 const v_2 = int3(z, z, z);

That would work here too, but would require changes to the IR that's
generated for (at least) this particular case, where a constructor is
generated to convert packed_uint3 to uint3. It seems more complicated to
make sure that fully covers all cases, whereas just inserting
parentheses here is simple.

Tests updated with:
  ./tools/run fix-tests out/r/tint_unittests
  ./tools/run tests --tint out/r/tint --generate-expected

Test: test/tint/statements/assign/phony
Test: ShaderTests.StorageAcrossStagesSameBindingPointCollideMixedStructDef
Test: ShaderTests.UniformAcrossStagesSameBindingPointCollideMixedStructDef
Bug: 363031535
Change-Id: I0999092a15a0ed584a39724ef342f9653478ba42
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/308297
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
SLSA-Policy-Verified: SLSA Policy Verification Service <devtools-gerritcodereview-exitgate@google.com>