[spec] Disallow throwing qualified objects (#3476)

See dlang/dmd#14706

Author: ntrel

spec/expression.dd

@@ -1484,6 +1484,11 @@ $(P
     throw new Exception("message");
     ---
 
+$(P
+    The `Throwable` must not be a qualified as `immutable`, `const`, `inout` or
+    `shared`. The runtime may modify a thrown object (e.g. to contain a stack
+    trace) which would violate `const` or `immutable` objects.
+)
 $(P
     A $(I ThrowExpression) may be nested in another expression:
 )