@_preInverseGenerics(except: <inverses>) is an extension of the existing
@_preInverseGenerics attribute that provides selective control over which
inverse requirements are mangled into a declaration's symbol name.
While the bare @_preInverseGenerics strips all inverse constraints
(~Copyable and ~Escapable) from mangling, the 'except:' form allows specific
inverses to be retained. This is needed when a type like Span already had
~Copyable mangled into its ABI-stable symbols and now needs to retroactively
adopt ~Escapable without changing those existing symbols. You can now express
that with `@_preInverseGenerics(except: ~Copyable)` to strip-out every inverse
except ~Copyable to preserve the pre-existing ~Copyable-containing symbols.
It requires the new experimental feature `PreInverseGenericsExcept`.
rdar://176395527
Kavon Farvardin