[MLIR] Primitive linkage lowering of FuncOp

FuncOp always lowers to an LLVM external linkage presently. This makes it impossible to define functions in mlir which are local to the current module. Until MLIR FuncOps have a more formal linkage specification, this commit allows funcop's to have an optionally specified llvm.linkage attribute, whose value will be used as the linkage of the llvm funcop when lowered.

Differential Revision: https://reviews.llvm.org/D108524

Support LLVM linkage