[flang][driver] Improve diagnostic for assembly inputs
When an assembly file is passed to the Flang driver, it currently fails
with a confusing internal error mentioning the unsupported `-cc1as`
tool.
This update detects assembly inputs early in the driver and emits a
clear, user-facing diagnostic explaining that Flang does not support
assembly files as input. The logic is implemented in a small helper
function and is intentionally temporary, so it can be removed once Flang
gains support for assembling inputs.
Fixes: #173528
