[mlir] LLVMFuncOp: provide a capability to pass attributes through to LLVM IR

Summary:
LLVM IR functions can have arbitrary attributes attached to them, some of which
affect may affect code transformations. Until we can model all attributes
consistently, provide a pass-through mechanism that forwards attributes from
the LLVMFuncOp in MLIR to LLVM IR functions during translation. This mechanism
relies on LLVM IR being able to recognize string representations of the
attributes and performs some additional checking to avoid hitting assertions
within LLVM code.

Differential Revision: https://reviews.llvm.org/D77072