[AMDGPU][HIP] Change default DWARF version to 4

Summary:
Tooling around DWARF 5 is still not mature enough for this to be a sane
default, and the AMDGPU and HIP toolchains should agree on a single
default.

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, aprantl, dstuttard, tpr, t-tye, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70191