bugfix: proper cache for generic instantiations

tests/accept/run/tgenericprocvar.nim

@@ -0,0 +1,19 @@
+discard """
+  output: "0false"
+"""
+
+# Test multiple generic instantiation of generic proc vars:
+
+proc threadProcWrapper[TMsg]() =
+  var x: TMsg
+  stdout.write($x)
+
+#var x = threadProcWrapper[int]
+#x()
+
+#var y = threadProcWrapper[bool]
+#y()
+
+threadProcWrapper[int]()
+threadProcWrapper[bool]()
+