Deprecate -fheinous-gnu-extensions; introduce a new warning flag (#105821)

The new warning flag is `-Winvalid-gnu-asm-cast`, which is enabled by
default and is a downgradable diagnostic which defaults to an error.

This language dialect flag only controls whether a single diagnostic is
emitted as a warning or as an error, and has never been expanded to
include other behaviors. Given the rather perjorative name, it's better
for us to just expose a diagnostic flag for the one warning in question
and let the user elect to do `-Wno-error=` if they need to.

There's not a lot of use of the language dialect flag in the wild, but
there is some use of it. For the time being, this aliases the -f flag to
`-Wno-error=invalid-gnu-asm-cast`, but the -f flag can eventually be
removed.

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