Ensure owner and receiver are reset after executing alias (#373)

This fixes an issue where the frame's owner/receiver are not reset
if a type test on a typealias fails.

pkl-core/src/test/files/LanguageSnippetTests/input/types/typeAlias2.pkl

@@ -35,3 +35,10 @@ typealias Bar = Foo
 class Baz extends Bar {}
 
 res7 = (new Baz {}).value
+
+typealias MappingOrListing = Mapping|Listing
+
+res8 = new {
+  foo = 5
+  bar = if (foo is MappingOrListing) "bar" else foo
+}

pkl-core/src/test/files/LanguageSnippetTests/output/types/typeAlias2.pcf

@@ -11,3 +11,7 @@ res6 {
   name = "Other"
 }
 res7 = 42
+res8 {
+  foo = 5
+  bar = 5
+}