[clang] Add support for “regular” keyword attributes

Platform-specific language extensions often want to provide a way of
indicating that certain functions should be called in a different way,
compiled in a different way, or otherwise treated differently from a
“normal” function.  Honoring these indications is often required for
correctness, rather being than an optimization/QoI thing.

If a function declaration has a property P that matters for correctness,
it will be ODR-incompatible with a function that does not have property P.
If a function type has a property P that affects the calling convention,
it will not be two-way compatible with a function type that does not
have property P.  These properties therefore affect language semantics.
That in turn means that they cannot be treated as standard [[]]
attributes.

Until now, many of these properties have been specified using GNU-style
attributes instead.  GNU attributes have traditionally been more lax
than standard attributes, with many of the...