[LLVM-C] Fill Out LLVMCallConv

Summary: Bring LLVMCallConv up to date with latest [[ http://llvm.org/doxygen/CallingConv_8h_source.html | CallConv ]]

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45347

llvm-svn: 329373