Correctly hash inferred types (#8286)

We don't really want to hash the tyInferred container since that'd make
the "real" type and its inferred counterpart produce different hashes
and types for the very same type.

Fixes #8280

compiler/sighashes.nim

@@ -163,7 +163,7 @@ proc hashType(c: var MD5Context, t: PType; flags: set[ConsiderFlag]) =
         c.hashType t.sons[i], flags
     else:
       c.hashType t.lastSon, flags
-  of tyAlias, tySink, tyUserTypeClasses:
+  of tyAlias, tySink, tyUserTypeClasses, tyInferred:
     c.hashType t.lastSon, flags
   of tyBool, tyChar, tyInt..tyUInt64:
     # no canonicalization for integral types, so that e.g. ``pid_t`` is

tests/concepts/t8280.nim

@@ -0,0 +1,16 @@
+discard """
+  output: "()"
+"""
+
+type
+  Iterable[T] = concept x
+    for elem in x:
+      elem is T
+
+proc max[A](iter: Iterable[A]): A = 
+  discard
+
+type
+  MyType = object
+
+echo max(@[MyType()])