Add regression test for #15709

Closes #15709

Author: nicolasstucki

tests/pos-macros/i15709.scala

@@ -0,0 +1,4 @@
+import quoted.*
+
+inline def foo(s: Singleton): Unit = ${ fooImpl('s) }
+def fooImpl(s: Expr[Singleton])(using Quotes) = '{}