Improve testing for the C API

Summary:
This basically add an echo test case in C. The support is limited right now, but full support would just be too much to review at once.

The echo test case simply get a module as input and try to output the same exact module. This allow to check the both reading and writing API are working as expected.

I want to improve this test over time to support more and more of the API, in order to improve coverage (coverage is quite poor right now).

Test Plan: Run the test.

Reviewers: chandlerc, bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10725

llvm-svn: 259844