Remove .bc from test config.suffixes

This was unused. Besides, a binary file can't really contain lit ShTests.
Add test coverage for .bc input in the frontend

Test that we can consume LLVM bitcode and additionally check that it produces
the same output as a direct compilation.

The feature is crashy and has gone untested until now, but we might as well
provide some coverage as long as it remains in tree.

Also test LL source input in the same way because the existing tests for that
don't look convincing.