[LLVM-C] Redo unnamed_address attribute bindings

Summary:
The old bindings should have used an enum instead of a boolean.  This
deprecates LLVMHasUnnamedAddr and LLVMSetUnnamedAddr , replacing them
with LLVMGetUnnamedAddress and LLVMSetUnnamedAddress respectively that do.
Though it is unlikely LLVM will gain more supported global value linker
hints, the new API can scale to accommodate this.

Reviewers: deadalnix, whitequark

Reviewed By: whitequark

Subscribers: llvm-commits, harlanhaskins

Differential Revision: https://reviews.llvm.org/D43448

llvm-svn: 327479