CI segfaults on older LLVM versions

[TheDan64]

CI will segfault on older LLVM versions

[TheDan64]

I've narrowed it down to LLVM versions 4-9, test_values.rs test_value_from_string test

[TheDan64]

This line, which makes sense:

f64_type.const_float_from_string(""). Need to see if there's some error handling from LLVM here that we're missing

[TheDan64]

LLVM does not appear to notify us of errors here :/

[TheDan64]

The LLVM method doesn't return a null ptr on error; the best we can do is maybe mark the wrapper as unsafe...

[TheDan64]

According to https://github.com/bugsnag/llvm/blob/79a45b8b272aadab6d8259ffae19903205a19c51/lib/Support/APFloat.cpp#L2588

an assert triggers on empty string, would be easy enough to check for but not nearly enough