tint: Deprecated module-scope 'let' for 'const'

Enable the parsing of 'const'.
Warn on use of module-scope 'let', and automatically replace with 'const'.

Fixed: tint:1580
Change-Id: I214aabca80686dc6b60ae21a7a57fbfb4898ea83
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/93786
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/tint/bug/chromium/1221120.wgsl

@@ -1,2 +1,2 @@
-let
+const
 H=1;

test/tint/bug/chromium/1221120.wgsl.expected.glsl

@@ -4,4 +4,3 @@ layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {
   return;
 }
-const int H = 1;

test/tint/bug/chromium/1221120.wgsl.expected.hlsl

@@ -3,4 +3,3 @@ void unused_entry_point() {
   return;
 }
 
-static const int H = 1;

test/tint/bug/chromium/1221120.wgsl.expected.msl

@@ -1,5 +1,3 @@
 #include <metal_stdlib>
 
 using namespace metal;
-constant int H = 1;
-

test/tint/bug/chromium/1221120.wgsl.expected.spvasm

@@ -1,19 +1,16 @@
 ; SPIR-V
 ; Version: 1.3
 ; Generator: Google Tint Compiler; 0
-; Bound: 7
+; Bound: 5
 ; Schema: 0
                OpCapability Shader
                OpMemoryModel Logical GLSL450
                OpEntryPoint GLCompute %unused_entry_point "unused_entry_point"
                OpExecutionMode %unused_entry_point LocalSize 1 1 1
-               OpName %H "H"
                OpName %unused_entry_point "unused_entry_point"
-        %int = OpTypeInt 32 1
-          %H = OpConstant %int 1
        %void = OpTypeVoid
-          %3 = OpTypeFunction %void
-%unused_entry_point = OpFunction %void None %3
-          %6 = OpLabel
+          %1 = OpTypeFunction %void
+%unused_entry_point = OpFunction %void None %1
+          %4 = OpLabel
                OpReturn
                OpFunctionEnd

test/tint/bug/chromium/1221120.wgsl.expected.wgsl

@@ -1 +1 @@
-let H = 1;
+const H = 1;