Unit tests to improve code coverage

Adds a batch of C tests that have been found to cover several hundred
lines of Clang/LLVM that are not covered by the unit and regression
tests of the main LLVM project, nor by the test suite when run with the
-O3 configuration.

The tests were originally generated using our fuzzer, and were then
reduced using C-Reduce and some manual inspection. They have been
checked for undefined behaviour-freedom using Frama-C and CompCert, and
manually checked to eliminate implementation-defined behaviour.

Differential Revision: https://reviews.llvm.org/D118234