tint: Add type constructors and converters support to intrinsic-gen

These are currently not used, but the first step towards moving type
constructors and converters over to using the intrinisc table.

This will simplify maintenance of type functions, and will greatly
simplify the [AbstractInt -> i32|u32] [AbstractFloat -> f32|f16] logic.

Bug: tint:1504
Change-Id: I15526670a6ff801e66551ab5adc37b1570ac49de
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/90242
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

tools/src/cmd/intrinsic-gen/sem/sem.go

@@ -22,13 +22,14 @@ import (
 
 // Sem is the root of the semantic tree
 type Sem struct {
-	Enums           []*Enum
-	Types           []*Type
-	TypeMatchers    []*TypeMatcher
-	EnumMatchers    []*EnumMatcher
-	Builtins        []*Intrinsic
-	UnaryOperators  []*Intrinsic
-	BinaryOperators []*Intrinsic
+	Enums                     []*Enum
+	Types                     []*Type
+	TypeMatchers              []*TypeMatcher
+	EnumMatchers              []*EnumMatcher
+	Builtins                  []*Intrinsic
+	UnaryOperators            []*Intrinsic
+	BinaryOperators           []*Intrinsic
+	ConstructorsAndConverters []*Intrinsic
 	// Maximum number of open-types used across all builtins
 	MaxOpenTypes int
 	// Maximum number of open-numbers used across all builtins