Make sure global const delcarations are not misreported as unused. (F…

…uelLabs#1537)

ConstantDeclarations aren't actually a thing as they're now all just
VariableDeclarations with 'pub const' mutability.  This needed to be
accounted for in the dead code analysis.

sway-core/src/control_flow_analysis/dead_code_analysis.rs

@@ -8,7 +8,7 @@ use crate::{
             TypedEnumDeclaration, TypedExpression, TypedExpressionVariant,
             TypedFunctionDeclaration, TypedReassignment, TypedReturnStatement,
             TypedStructDeclaration, TypedStructExpressionField, TypedTraitDeclaration,
-            TypedVariableDeclaration, TypedWhileLoop,
+            TypedVariableDeclaration, TypedWhileLoop, VariableMutability,
         },
         TypeCheckedStorageReassignment, TypedAstNode, TypedAstNodeContent, TypedParseTree,
     },
@@ -338,15 +338,27 @@ fn connect_declaration(
 ) -> Result<Vec<NodeIndex>, CompileError> {
     use TypedDeclaration::*;
     match decl {
-        VariableDeclaration(TypedVariableDeclaration { body, .. }) => connect_expression(
-            &body.expression,
-            graph,
-            &[entry_node],
-            exit_node,
-            "variable instantiation",
-            tree_type,
-            body.clone().span,
-        ),
+        VariableDeclaration(TypedVariableDeclaration {
+            name,
+            body,
+            is_mutable,
+            ..
+        }) => {
+            if matches!(is_mutable, VariableMutability::ExportedConst) {
+                graph.namespace.insert_constant(name.clone(), entry_node);
+                Ok(leaves.to_vec())
+            } else {
+                connect_expression(
+                    &body.expression,
+                    graph,
+                    &[entry_node],
+                    exit_node,
+                    "variable instantiation",
+                    tree_type,
+                    body.clone().span,
+                )
+            }
+        }
         ConstantDeclaration(TypedConstantDeclaration { name, .. }) => {
             graph.namespace.insert_constant(name.clone(), entry_node);
             Ok(leaves.to_vec())