Introduce Linkage attribute to the LLVM dialect

LLVM IR supports linkage on global objects such as global variables and
functions. Introduce the Linkage attribute into the LLVM dialect, backed by an
integer storage. Use this attribute on LLVM::GlobalOp and make it mandatory.
Implement parsing/printing of the attribute and conversion to LLVM IR.

See tensorflow/mlir#277.

PiperOrigin-RevId: 283309328