[clang] [docs][HIP] Document source-based device code coverage workflow (PR #200197)

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

Hi, could you please take a look when you get a chance?

The offload PGO support PRs have been relanded for a little while now, and things seem to have stabilized. This PR documents the user-facing HIP device code coverage workflow. CI is green.

https://github.com/llvm/llvm-project/pull/200197