I made two major improvements to the way the

master AST importer imports types.

- First, before importing the definition of a
  Decl from its source, notify the underlying
  importer of the source->destination mapping.
  Especially for anonymous strucutres that are
  otherwise hard to unique in the target AST
  context, this hint is very helpful.

- When deporting a type or Decl from one
  ASTContext to another (deporting occurs in
  the case of moving result types from the
  parser's AST context to the result AST
  context), don't forget their origin if the
  origin is the original debug information.

llvm-svn: 148152