[DebugInfo] Handle missed DW_FORM_addrx3 and DW_FORM_strx3 cases

This fixes a few places where the addrx3 and strx3 forms were missed.
Previously this meant if one of these forms appeared somewhere various
errors could occur. This now also adds an extra test case for the addrx3
form (which previously failed).

Differential Revision: https://reviews.llvm.org/D143488