Add type inference support to Resolver

There is still no way to spell this out in WGSL, but this adds support
for VariableDecls with an ast::Variable that has nullptr type. In this
case, the Resolver uses the type of the rhs (constructor expression),
which is stored in semantic::Variable.

Added tests for resolving inferred types from constructor, arithmetic,
and call expressions.

Bug: tint:672
Change-Id: I3dcfd18adecebc8b969373d2ac72c21891c21a87
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/46160
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

src/ast/variable.cc

@@ -38,7 +38,8 @@ Variable::Variable(const Source& source,
       decorations_(std::move(decorations)),
       declared_storage_class_(declared_storage_class) {
   TINT_ASSERT(symbol_.IsValid());
-  TINT_ASSERT(declared_type_);
+  // no type means we must have a constructor to infer it
+  TINT_ASSERT(declared_type_ || constructor);
 }
 
 Variable::Variable(Variable&&) = default;