[clang] [clang][docs] Document language selection and high-level driver options (PR #213542)

[via cfe-commits]

basisworks wrote:

Read all three, and the AI tool use policy in particular.

This is an AI-assisted contribution and it is labelled as such. I have changed the line at the end of the description to the `Assisted-by:` trailer the policy suggests, since that is the form it asks for.

On the substance, the new section was written against the driver sources rather than the older prose:

- the extension table is from `lookupTypeForExtension` in `clang/lib/Driver/Types.cpp`
- the `-x` behaviour (applies to inputs *after* it, persists until the next `-x`, `-x none` restores inference) is from `Driver::BuildInputs` in `clang/lib/Driver/Driver.cpp`, where `TY_Nothing` is what means "infer from the extension"

That is why a few entries differ from what the manual currently implies, particularly the module-interface, HLSL and CUDA/HIP extensions.

92 lines is on the large side for a first contribution here. If reviewers would rather it were split, or cut back to just the `-x` and stage-selection parts and the rest dropped, I am happy to do that -- the dangling "the first part introduces..." sentence in #10796 is the thing worth fixing, and a smaller patch closes it just as well.

https://github.com/llvm/llvm-project/pull/213542