Move tint::ast::type to tint::type

Despite `tint::ast::type::Type` being in the AST namespace, these classes are clearly not AST nodes:
* They don't derive from ast::Node
* They're deduplicated by the type manager
* None of the types have an Source - they have no lexical declaration point
* The fact we have `ast::Struct` and `ast::type::Struct` clearly demonstrates what is an AST node, and what is a type.
* We have code scattered in the codebase (TypeDeterminer, writers, etc) that create new types after parsing - so clearly not part of the original syntax tree.

Types in tint are closer to being semantic info, but due to the parse-time generation of types, and tight dependency of ast::Nodes to types, I'd be reluctant to class these as semantic info. Instead, put these into a separate root level `tint::type` namespace and `src/tint` directory.

The fact that types exist in the ast::Module has already caused bugs (https://dawn-review.googlesource.com/c/tint/+/37261). This is a first step in separating out types from the ast::Module.

Bug: tint:390
Change-Id: I8349bbbd1b19597b8e6d51d5cda0890de46ecaec
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38002
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

include/tint/tint.h

@@ -19,7 +19,6 @@
 //                headers will need to be moved to include/tint/.
 
 #include "src/ast/pipeline_stage.h"
-#include "src/ast/type_manager.h"
 #include "src/demangler.h"
 #include "src/diagnostic/printer.h"
 #include "src/inspector/inspector.h"
@@ -30,6 +29,7 @@
 #include "src/transform/first_index_offset.h"
 #include "src/transform/manager.h"
 #include "src/transform/vertex_pulling.h"
+#include "src/type/type_manager.h"
 #include "src/type_determiner.h"
 #include "src/validator/validator.h"
 #include "src/writer/writer.h"