[libc] Add integration tests.

Summary:
This patch aims to add integration tests to check the following:
1) Header files are generated as expected.
2) Libc functions have the correct public name.
3) Libc functions have the correct return type and parameter types.
4) Symbols are exposed in the public lib.a files.

Reviewers: sivachandra, abrachet

Reviewed By: sivachandra

Subscribers: aheejin, ecnelises, dxf, mgorny, jfb, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D79192