Rename Minion to ASTImporterDelegate

Summary:
I think there universal agreement that Minion isn't the best name for this class. This patch renames the class
 to ASTImporterDelegate to better reflect it's goal of monitoring and extending the ASTImporter.

Reviewers: aprantl, shafik, martong, a.sidorin, davide

Reviewed By: aprantl, shafik, davide

Subscribers: rnkovacs, davide, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61299

llvm-svn: 359777