lang/funcs: lookup() will only treat map as unknown if it is wholly unknown (#26427)

Fix for issue #26320 - this allows us to derive known values from
partially known maps where we can, and may prevent unnecessary
destroy/rebuild cycles during apply in some cases.

lang/funcs/collection.go

@@ -315,7 +315,7 @@ var LookupFunc = function.New(&function.Spec{
 		mapVar := args[0]
 		lookupKey := args[1].AsString()
 
-		if !mapVar.IsWhollyKnown() {
+		if !mapVar.IsKnown() {
 			return cty.UnknownVal(retType), nil
 		}
 

lang/funcs/collection_test.go

@@ -720,6 +720,14 @@ func TestLookup(t *testing.T) {
 			cty.UnknownVal(cty.String),
 			false,
 		},
+		{
+			[]cty.Value{
+				mapWithUnknowns,
+				cty.StringVal("foo"),
+			},
+			cty.StringVal("bar"),
+			false,
+		},
 		{
 			[]cty.Value{
 				simpleMap,