Optionally add code completion results for arrow instead of dot

Currently getting such completions requires source correction, reparsing
and calling completion again. And if it shows no results and rollback is
required then it costs one more reparse.

With this change it's possible to get all results which can be later
filtered to split changes which require correction.

Differential Revision: https://reviews.llvm.org/D41537

llvm-svn: 333272