survive invalid .ast files

previously died with Assertion error
fix incorrect propagation of fexpr context

potentially reported errors in a too wide context
decouple CDeclUse from the type system to make it optional
Merge remote-tracking branch 'upstream/master'

Conflicts:
	CTypeChecker/src/main/scala/de/fosd/typechef/typesystem/CDeclUse.scala
	CTypeChecker/src/main/scala/de/fosd/typechef/typesystem/CExprTyping.scala
	CTypeChecker/src/main/scala/de/fosd/typechef/typesystem/CTypeEnv.scala
	Frontend/src/main/scala/de/fosd/typechef/Frontend.scala