Discriminate gensym'd type names in sigHash

The root cause of #7905 lies in the codegen phase. The two template
instantiations generate two different MyType types with different
members but same t.sym.name leading the caching mechanism to confuse
the two.

Fixes #7905

tests/types/t7905.nim

@@ -0,0 +1,18 @@
+discard """
+  output: '''
+(member: "hello world")
+(member: 123.456)
+'''
+"""
+
+template foobar(arg: typed): untyped =
+  type
+    MyType = object
+      member: type(arg)
+
+  var myVar: MyType
+  myVar.member = arg
+  echo myVar
+
+foobar("hello world")
+foobar(123.456'f64)