Allow array size to be a module-scope constant

Change ast::Array to use an ast::Expression for its `size` field. The
WGSL frontend now parses the array size as an `primary_expression`,
and the Resolver is responsible for validating the expression is a
signed or unsigned integer, and either a literal or a non-overridable
module-scope constant.

The Resolver evaluates the constant value of the size expression, and
so the resolved sem::Array type still has a constant size as before.

Fixed: tint:1068
Fixed: tint:1117

Change-Id: Icfa141482ea1e47ea8c21a25e9eb48221f176e9a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/63061
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

test/array/size.wgsl

@@ -0,0 +1,14 @@
+let slen = 4;
+let ulen = 4u;
+
+[[stage(fragment)]]
+fn main() {
+  var signed_literal : array<f32, 4>;
+  var unsigned_literal : array<f32, 4u>;
+  var signed_constant : array<f32, slen>;
+  var unsigned_constant : array<f32, ulen>;
+
+  // Ensure that the types are compatible.
+  signed_literal = unsigned_constant;
+  signed_constant = unsigned_literal;
+}

test/array/size.wgsl.expected.hlsl

@@ -0,0 +1,12 @@
+static const int slen = 4;
+static const uint ulen = 4u;
+
+void main() {
+  float signed_literal[4] = (float[4])0;
+  float unsigned_literal[4] = (float[4])0;
+  float signed_constant[4] = (float[4])0;
+  float unsigned_constant[4] = (float[4])0;
+  signed_literal = unsigned_constant;
+  signed_constant = unsigned_literal;
+  return;
+}

test/array/size.wgsl.expected.msl

@@ -0,0 +1,19 @@
+#include <metal_stdlib>
+
+using namespace metal;
+struct tint_array_wrapper {
+  float arr[4];
+};
+
+constant int slen = 4;
+constant uint ulen = 4u;
+fragment void tint_symbol() {
+  tint_array_wrapper signed_literal = {};
+  tint_array_wrapper unsigned_literal = {};
+  tint_array_wrapper signed_constant = {};
+  tint_array_wrapper unsigned_constant = {};
+  signed_literal = unsigned_constant;
+  signed_constant = unsigned_literal;
+  return;
+}
+

test/array/size.wgsl.expected.spvasm

@@ -0,0 +1,39 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 19
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %main "main"
+               OpExecutionMode %main OriginUpperLeft
+               OpName %slen "slen"
+               OpName %ulen "ulen"
+               OpName %main "main"
+               OpName %signed_literal "signed_literal"
+               OpName %unsigned_literal "unsigned_literal"
+               OpName %signed_constant "signed_constant"
+               OpName %unsigned_constant "unsigned_constant"
+               OpDecorate %_arr_float_ulen ArrayStride 4
+        %int = OpTypeInt 32 1
+       %slen = OpConstant %int 4
+       %uint = OpTypeInt 32 0
+       %ulen = OpConstant %uint 4
+       %void = OpTypeVoid
+          %5 = OpTypeFunction %void
+      %float = OpTypeFloat 32
+%_arr_float_ulen = OpTypeArray %float %ulen
+%_ptr_Function__arr_float_ulen = OpTypePointer Function %_arr_float_ulen
+         %13 = OpConstantNull %_arr_float_ulen
+       %main = OpFunction %void None %5
+          %8 = OpLabel
+%signed_literal = OpVariable %_ptr_Function__arr_float_ulen Function %13
+%unsigned_literal = OpVariable %_ptr_Function__arr_float_ulen Function %13
+%signed_constant = OpVariable %_ptr_Function__arr_float_ulen Function %13
+%unsigned_constant = OpVariable %_ptr_Function__arr_float_ulen Function %13
+         %17 = OpLoad %_arr_float_ulen %unsigned_constant
+               OpStore %signed_literal %17
+         %18 = OpLoad %_arr_float_ulen %unsigned_literal
+               OpStore %signed_constant %18
+               OpReturn
+               OpFunctionEnd

test/array/size.wgsl.expected.wgsl

@@ -0,0 +1,13 @@
+let slen = 4;
+
+let ulen = 4u;
+
+[[stage(fragment)]]
+fn main() {
+  var signed_literal : array<f32, 4>;
+  var unsigned_literal : array<f32, 4u>;
+  var signed_constant : array<f32, slen>;
+  var unsigned_constant : array<f32, ulen>;
+  signed_literal = unsigned_constant;
+  signed_constant = unsigned_literal;
+}