Added support to the ASTImporter for passing

completion information between different AST
contexts.  It works like this:

- If a Decl is imported from a context that
  has completion metadata, then that Decl
  is associated with the same completion
  information (possibly none) as the Decl
  it was imported from.

- If a Decl is imported from a context that
  does not have completion metadata, then it
  is marked as completable by consulting the
  Decl and context it was imported from.

llvm-svn: 144838