Improve the C API echo test tool to emit basic block is the right order.

Summary: As per title. Also add a facility method to get the name of a basic block from the C API.

Reviewers: bogner, chandlerc, echristo, dblaikie, joker.eph, Wallbraker

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16912

llvm-svn: 260309