[clang] [Driver] Teach Barmetal toolchain about GCC installation (PR #121829)

[Garvit Gupta via cfe-commits]

quic-garvgupt wrote:

> I've done another review pass. Mostly some small stylistic comments.
> 
> I'm finding it difficult to follow the code with the various different code-paths with similar names. I've made some suggestions on renaming. We may also be able to add some more comments to make it clearer.

I've made few more refactoring changes in addition to suggestion to improve code readability and facilitate the review process:
- Renamed the function from `hasGCCToolchainAlongSideClang` to `detectGCCToolchainAdjacent`, and added a comment explaining its purpose. (Pls let me know if there is need for a `fixme` or `todo` here to remove/fix this logic in future.)
- In the `computeSysRoot` function, renamed the local variable `SysRootDir` to `inferredSysRoot` for better clarity, and added additional comments to explain the logic.
- In the constructor, grouped all file path additions together and all program path additions together, with comments explaining the rationale behind each block.

https://github.com/llvm/llvm-project/pull/121829