Repurpose mangling operator `Y` as an umbrella operator that covers new attributes on function types. Free up operators `J`, `j`, and `k`.
```
async ::= 'Ya' // 'async' annotation on function types
sendable ::= 'Yb' // @Sendable on function types
throws ::= 'K' // 'throws' annotation on function types
differentiable ::= 'Yjf' // @differentiable(_forward) on function type
differentiable ::= 'Yjr' // @differentiable(reverse) on function type
differentiable ::= 'Yjd' // @differentiable on function type
differentiable ::= 'Yjl' // @differentiable(_linear) on function type
```
Resolves rdar://76299796.
We're not quite ready to commit to the flow-sensitive check that would
allow a concurrent function to read from a mutable local capture so
long as the captured variable wasn't changed after the point of
capture. Put it behind a flag and implement the more restrictive rule
(no access to mutable local captures in concurrent code). We can relax
it later.
Add @concurrent to SIL function types, mirroring what's available on
AST function types. @concurrent function types will have by-value
capture semantics.
