[DebugInfo] Handle generic type DW_OP_convert ops in llvm-dwarfdump

Summary:
This is a preparatory change for allowing LLVM to emit DW_OP_convert
operations converting to the generic type.

If DW_OP_convert's operand is 0, it converts the top of stack to the
generic type, as specified by DWARFv5 section 2.5.1.6:

"[...] takes one operand, which is an unsigned LEB128 integer that
 represents the offset of a debugging information entry in the current
 compilation unit, or value 0 which represents the generic type."

This adds support for such operations to llvm-dwarfdump.

Reviewers: aprantl, markus, jdoerfert, jhenderson

Reviewed By: aprantl

Subscribers: hiraditya, llvm-commits

Tags: #debug-info, #llvm

Differential Revision: https://reviews.llvm.org/D76141