TranslationUnit now owns IdentifierTable, TargetInfo, and Selectors objects

when it is constructed via deserialization.  This is done by recording a flag
indicating that this is the case, and it deletes these objects by getting
the references stored in the ASTContext object.  This fixes some memory
leaks that occurs when we deserialize translation units from bitcode files.

The rationale between having TranslationUnit sometimes own these objects and
sometimes not is that a TranslationUnit object can be constructed from
state generated by the parser (Preprocessor; semantic analyzer, etc.), and thus
in these cases won't own the IdentifierTable or Selectors, etc.  During
deserialization, there is no Preprocessor, so somebody needs to own these
objects in order for them to be properly reclaimed.

llvm-svn: 50149