Enable XFAIL'ed bounds widening unit tests

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#1109

---

In checkedc/checkedc-clang#1108 I have XFAIL'ed the following bounds widening tests that need to be re-enabled once we implement features that the tests need.

1. test/CheckedC/inferred-bounds/widened-bounds-strings-examples.c --> This can be enabled once we have implemented invertibility support and bounds can be widened in a loop.

2. test/CheckedC/inferred-bounds/widened-bounds-check.c --> This can be enabled once we have implemented Available Facts Analysis that will give us the current bounds of a pointer so that we know the bounds to be set for pointers whose bounds have been killed.