ast: Remove TypeConstructorExpression

Add a new 'Target' to the ast::CallExpression, which can be either an
Identifier or Type. The Identifier may resolve to a Type, if the Type is
a structure or alias.

The Resolver now resolves the CallExpression target to one of the
following sem::CallTargets:
* sem::Function
* sem::Intrinsic
* sem::TypeConstructor
* sem::TypeCast

This change will allow us to remove the type tracking logic from the WGSL
parser, which is required for out-of-order module scope declarations.

Bug: tint:888
Bug: tint:1266
Change-Id: I696f117115a50981fd5c102a0d7764641bb755dd
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/68525
Reviewed-by: David Neto <dneto@google.com>
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/BUILD.gn

@@ -203,7 +203,6 @@ tint_unittests_source_set("tint_unittests_ast_src") {
     "../src/ast/test_helper.h",
     "../src/ast/texture_test.cc",
     "../src/ast/traverse_expressions_test.cc",
-    "../src/ast/type_constructor_expression_test.cc",
     "../src/ast/u32_test.cc",
     "../src/ast/uint_literal_expression_test.cc",
     "../src/ast/unary_op_expression_test.cc",