[libc] Add integration tests.
This patch aims to add integration tests to check the following:
- Header files are generated as expected.
- Libc functions have the correct public name.
- Libc functions have the correct return type and parameter types.
- Symbols are exposed in the public lib.a files.
Reviewers: sivachandra, abrachet
Reviewed By: sivachandra
Subscribers: aheejin, ecnelises, dxf, mgorny, jfb, tschuett, libc-commits
Differential Revision: https://reviews.llvm.org/D79192