[libc] Add documentation for integration tests.

[MaskRay]

libc/docs/integration_test.rst

18

Note that rst is different from MarkDown. This probably should be double apostrophes.

[PaulkaToast]

libc/docs/integration_test.rst

18

Thank you for catching this!

libc/docs/integration_test.rst

+Integration Tests
+=====================
+The implementation of libc-project is unique because our public C header files
+are generated using information from TableGen. Unit tests only exercise the
+internal C++ implementations and don't ensure the headers were generated by the
+build system. End to end testing ensures that, after building, the produced
+library and headers are usable in the C runtime. A simple solution is to have
+contributors write an integration test for each individual function as a C
+program; however, this would place a large burden on contributors and duplicates
+some effort from the unit tests.
+
+Instead we automate the generation of integration tests by modeling it from our
+generated headers. These integration tests ensure that public facing symbols are
+visible, that header files are generated as expected, and that each libc
+function has the correct function prototype.
+
+The integration test cmake rules are located in ``test/src/CMakeLists.txt`` and
+the generated integration test lives in
+``llvm/build/projects/libc/test/src/public_integration_test.cpp``