[LLVM-C] Allow Specifying Signedness in Int Cast

Summary: Fix an old outstanding problem with the int cast builder binding always assuming the cast is signed by introducing a new LLVMBuildIntCast2 operation and deprecating the old prototype.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56280

llvm-svn: 350607