[mlir][LLVM] Add an attribute to control use of bare-pointer calling convention.

Currently the use of bare pointer calling convention is controlled
globally through use of an option in the `LLVMTypeConverter`. To allow
more fine-grained control use an attribute on a function to drive the
calling convention to use.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D147494