[DebugInfo] Prep llvm-dwarfdump for typed DW5 ops.

Adds llvm-dwarfdump support for pretty printing Dwarf5 expressions ops
that reference a base type (right now only DW_OP_convert is added).
Includes verification to verify that the ops operand is actually a
DW_TAG_base_type DIE.

Differential Revision: https://reviews.llvm.org/D58442

llvm-svn: 354552