Simplify warning flag value handling from r206826

Also give the field it a more appropriate name and improve the docs.
clang-interpreter: don't check input file existence, we're in-process

This flag is set by most other tools and avoids extra stat() calls. The
frontend will diagnose anyway as it performs the check atomically while opening
files at point of use.

We could probably make Driver::CheckInputsExist default to false and only
enable it in the main 'clang' binary, or even better only perform the checks if
we know the tool is external but that needs more thought.