Move decl completion out of the ASTImporterDelegate and document it [NFC]

Summary:
The ASTImporterDelegate is currently responsible for both recording and also completing
types. This patch moves the actual completion and recording code outside the ASTImporterDelegate
to reduce the amount of responsibilities the ASTImporterDelegate has to fulfill.

As I anyway had to touch the code when moving I also documented and refactored most of it
(e.g. no more asserts that we call the deporting start/end function always as a pair).

Note that I had to make the ASTImporterDelegate and it's related functions public now so that
I can move out the functionality in another class (that doesn't need to be in the header).

Reviewers: shafik, aprantl, martong, a.sidorin

Reviewed By: martong

Subscribers: rnkovacs, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61478

llvm-svn: 372385