Allow global enum definitions.

test/libsolidity/syntaxTests/enums/global_enum.sol

@@ -0,0 +1,7 @@
+enum E { A }
+contract C {
+    function f() public pure {
+        E e = E.A;
+        e;
+    }
+}

test/libsolidity/syntaxTests/enums/global_enum_contract_name_clash.sol

@@ -0,0 +1,4 @@
+enum E { A }
+contract E {}
+// ----
+// DeclarationError: (13-26): Identifier already declared.

test/libsolidity/syntaxTests/enums/global_enum_name_clash.sol

@@ -0,0 +1,4 @@
+enum E { A }
+enum E { A }
+// ----
+// DeclarationError: (13-25): Identifier already declared.

test/libsolidity/syntaxTests/enums/global_enum_shadowing.sol

@@ -0,0 +1,10 @@
+enum E { A }
+contract C {
+    enum E { A }
+    function f() public pure {
+        E e = E.A;
+        e;
+    }
+}
+// ----
+// Warning: (30-42): This declaration shadows an existing declaration.

test/libsolidity/syntaxTests/imports/name_clash_in_import_enum.sol

@@ -0,0 +1,7 @@
+==== Source: a ====
+enum E { A }
+==== Source: b ====
+import "a";
+enum E { A }
+// ----
+// DeclarationError: (b:12-24): Identifier already declared.

test/libsolidity/syntaxTests/imports/name_clash_in_import_enum_contract.sol

@@ -0,0 +1,7 @@
+==== Source: a ====
+enum E { A }
+==== Source: b ====
+import "a";
+contract E { }
+// ----
+// DeclarationError: (b:12-26): Identifier already declared.

test/libsolidity/syntaxTests/imports/name_clash_in_import_enum_struct.sol

@@ -0,0 +1,7 @@
+==== Source: a ====
+enum E { A }
+==== Source: b ====
+import "a";
+struct E { uint256 a; }
+// ----
+// DeclarationError: (b:12-35): Identifier already declared.