[change-namespace] shorten namespace qualifier based on UsingDecl and UsingDirectiveDecl.

Summary:
when replacing symbol references in moved namespaces, trying to make the replace
name as short as possible by considering UsingDecl (i.e. UsingShadow) and
UsingDirectiveDecl (i.e. using namespace decl).

Reviewers: hokein

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25771

llvm-svn: 286307