llvmjit: Work around bug in LLVM 3.9 causing crashes after 72559438f92.

llvmjit: Work around bug in LLVM 3.9 causing crashes after 72559438f92.

Unfortunately in LLVM 3.9 LLVMGetAttributeCountAtIndex(func, index)
crashes when called with an index that has 0 attributes. Since there's
no way to work around this in the C API, add a small C++ wrapper doing
so.

The only reason this didn't fail before 72559438f92 is that there
always are function attributes...

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20201016001254.w2nfj7gd74jmb5in@alap3.anarazel.de
Backpatch: 11-, like 72559438f92