[mlir][llvm] Make some debug info attribute parameters optional.

[mlir][llvm] Make some debug info attribute parameters optional.

The revision makes specific debug information attribute parameters
optional since some of them can be omitted in LLVMIR. The additional
flexibility enables a later revision that will support importing
debug information from LLVMIR. A special case is the types parameter
of the SubroutineTypeAttr. For void functions, its first entry is
null in LLVMIR. This revision splits the type parameter in an optional
resultType parameter and an argumentTypes array to support this corner
case.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D138206