Don't parse lambda as UDA without parentheses (#21009)

This brings the compiler's behavior in line with the language spec.

Fixes dlang/dlang.org#4137

compiler/test/compilable/uda_lambda.d

@@ -0,0 +1,3 @@
+enum UDA;
+int fun() @UDA => 7;
+static assert(fun() == 7);

compiler/test/fail_compilation/uda_lambda.d

@@ -0,0 +1,7 @@
+/+
+TEST_OUTPUT:
+---
+fail_compilation/uda_lambda.d(7): Error: declaration expected, not `=>`
+---
++/
+@a => 7 int b;