Handle mixin expressions as assert conditions

For druntime's test/exceptions/src/assert_fail.d, which still needs a
few adaptations.

src/dmd/expressionsem.d

@@ -5821,6 +5821,12 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
             }()
             */
 
+            // if the assert condition is a mixin expression, try to compile it
+            if (auto ce = exp.e1.isCompileExp()) {
+              if (auto e1 = compileIt(ce))
+                exp.e1 = e1;
+            }
+
             const tok = exp.e1.op;
             bool isEqualsCallExpression;
             if (tok == TOK.call)