Allow global struct definitions.

test/libsolidity/syntaxTests/structs/contract_global_struct_name_clash.sol

@@ -0,0 +1,4 @@
+contract S {}
+struct S { uint256 a; }
+// ----
+// DeclarationError: (14-37): Identifier already declared.

test/libsolidity/syntaxTests/structs/global_struct.sol

@@ -0,0 +1,7 @@
+struct S { uint a; }
+contract C {
+    function f() public pure {
+        S memory s = S(42);
+        s;
+    }
+}

test/libsolidity/syntaxTests/structs/global_struct_contract_name_clash.sol

@@ -0,0 +1,4 @@
+struct S { uint256 a; }
+contract S {}
+// ----
+// DeclarationError: (24-37): Identifier already declared.

test/libsolidity/syntaxTests/structs/global_struct_shadowing.sol

@@ -0,0 +1,10 @@
+struct S { uint a; }
+contract C {
+    struct S { address x; }
+    function f() public view {
+        S memory s = S(address(this));
+        s;
+    }
+}
+// ----
+// Warning: (38-61): This declaration shadows an existing declaration.

test/libsolidity/syntaxTests/structs/global_structs_name_clash.sol

@@ -0,0 +1,4 @@
+struct S { uint256 a; }
+struct S { uint256 a; }
+// ----
+// DeclarationError: (24-47): Identifier already declared.