Make type lookup safer

[Jesper Öqvist]

The lookupType attribute can return null. The attribute should be made safer by instead returning UnknownType.

[Jesper Öqvist]

In most cases the result of lookupType is checked for nullness, however some standard type lookups are not null-checked, for example the ones below:

syn lazy TypeDecl Program.typeObject() = lookupType("java.lang", "Object");
syn lazy TypeDecl Program.typeCloneable() = lookupType("java.lang", "Cloneable");
syn lazy TypeDecl Program.typeSerializable() = lookupType("java.io", "Serializable");

Normally these types will be available, but if they for some reason are not (misconfigured boot classpath) then we get a NullPointerException.

[Jesper Öqvist]

Made lookupType non-null

Program.lookupType(String,String) now returns the UnknownType instance for failed type lookups.

fixes #46 (bitbucket)

→ <<cset 416320d9c17c>>