clang-rename: add a -old-name option

This is similar to -offset with the following differences:

1) -offset can refer to local variables as well.

2) -old-name makes it easier to refer to e.g. ClassName::MemberName by
spelling out the fully qualified name, instead of having to use e.g.
grep to look up the exact offset.

In other words, -offset is great when clang-rename is invoked by e.g. an
IDE, but not really user-friendly when the tool is invoked by the user
from commandline.  That's the use case where -old-name is supposed to
improve the situation.

Reviewers: klimek

Differential Revision: http://reviews.llvm.org/D21517

llvm-svn: 273304