[libclang] Return deduced type for auto type, not the one written in the source.

It used to work, but was accidentally broken by r179769.
The issue with decayed types was fixed by r190796.
So this patch partially reverts r179769, and adds more tests.

This also fixes PR 18669.

Patch by Sergey Kalinichev.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246778 91177308-0d34-0410-b5e6-96231b3b80d8