[LLVM-C] Fix ByVal Attribute crashing

With the introduction of the typed byval attribute change there was no
way that the LLVM-C API could create the correct class Attribute. If a
program that uses the C API creates a ByVal attribute and annotates a
function with that attribute LLVM will crash when it assembles or write
that module containing the function out as bitcode.

This change is a minimal fix to at least allow code to work, this is
because the byval change is on the 9.0 and I don't want to introduce new
LLVM-C API this late in the release cycle.

By Jakob Bornecrantz!

Differential revision: https://reviews.llvm.org/D66144

llvm-svn: 370176