[AST] Extract Decl::printNestedNameSpecifier helper from Decl::printQualifiedName

Summary:
To be used in clangd, e.g. in D66647.
Currently the alternative to this function is doing string manipulation on results of `printQualifiedName`, which is
hard-to-impossible to get right in presence of template arguments.

Reviewers: kadircet, aaron.ballman

Reviewed By: kadircet, aaron.ballman

Subscribers: aaron.ballman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67825

llvm-svn: 372863