Allow UserDefinedValueType.uwrap (and wrap) as RHS of constant decl

Needed to make `MyType.unwrap` and `MyType.unwrap` as pure in the process. This change affected some
existing tests ("statement has no effect").

test/libsolidity/semanticTests/userDefinedValueType/constant.sol

@@ -0,0 +1,13 @@
+type T is int224;
+pragma solidity >= 0.0.0;
+contract C {
+  T constant public s = T.wrap(int224(165521356710917456517261742455526507355687727119203895813322792776));
+  T constant public t = s;
+  int224 constant public u = T.unwrap(t);
+}
+// ====
+// compileViaYul: also
+// ----
+// s() -> 165521356710917456517261742455526507355687727119203895813322792776
+// t() -> 165521356710917456517261742455526507355687727119203895813322792776
+// u() -> 165521356710917456517261742455526507355687727119203895813322792776

test/libsolidity/syntaxTests/userDefinedValueType/constant.sol

@@ -0,0 +1,8 @@
+contract C {
+    type MyInt is int;
+    MyInt constant mi = MyInt.wrap(5);
+    // This is currently unsupported.
+    uint[MyInt.unwrap(mi)] arr;
+}
+// ----
+// TypeError 5462: (122-138): Invalid array length, expected integer literal or constant expression.

test/libsolidity/syntaxTests/userDefinedValueType/explicit_conversions_wrap.sol

@@ -6,3 +6,5 @@ function f() pure {
     MyAddress.wrap(address(5));
 }
 // ----
+// Warning 6133: (73-87): Statement has no effect.
+// Warning 6133: (93-119): Statement has no effect.

test/libsolidity/syntaxTests/userDefinedValueType/explicit_literal_conversion_wrap.sol

@@ -7,3 +7,6 @@ function f() pure {
     MyUInt8.wrap(50);
 }
 // ----
+// Warning 6133: (75-101): Statement has no effect.
+// Warning 6133: (107-122): Statement has no effect.
+// Warning 6133: (128-144): Statement has no effect.