[MLIR][SPIRVToLLVM] SPIR-V function call conversion pattern

Added conversion pattern for SPIR-V `FunctionCallOp`. Based on
specification, it returns no results or a single result, so
can be mapped directly to LLVM dialect's `llvm.call`.

Reviewed By: antiagainst, ftynse

Differential Revision: https://reviews.llvm.org/D83030