[clang-diff] Use the relative name for NamedDecl

[clang-diff] Use the relative name for NamedDecl

Summary:
If a node referring to a name is within a class or namespace, do not use
the full qualified name, but strip the namespace prefix.

Reviewers: arphaman, bkramer

Subscribers: klimek

Differential Revision: https://reviews.llvm.org/D36681

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311433 91177308-0d34-0410-b5e6-96231b3b80d8