Use the `%target-swift-5.1-abi-triple` substitution to compile the tests for
deployment to the minimum OS versions required for use of _Concurrency APIs,
instead of disabling availability checking.
Out of an abundance of caution, we:
1. Left in parsing support for transferring but internally made it rely on the
internals of sending.
2. Added a warning to tell people that transferring was going to
be removed very soon.
Now that we have given people some time, remove support for parsing
transferring.
rdar://130253724
We want to ensure that functions/methods themselves do not have sending mangled
into their names, but we do want sending mangled in non-top level positions. For
example: we do not want to mangle sending into a function like the following:
```swift
// We don't want to mangle this.
func test(_ x: sending NonSendableKlass) -> ()
```
But when it comes to actually storing functions into memory, we do want to
distinguish in between function values that use sending vs those that do not
since we do not want to allow for them to alias. Thus we want to mangle sending
into things like the following:
```swift
// We want to distinguish in between Array<(sending T) -> ()> and
// Array((T) -> ()>
let a = Array<(sending T) -> ()>
// We want to distinguish in between a global contianing (sending T) -> () and a
// global containing (T) -> ().
var global: (sending T) -> ()
```
This commit achieves that by making changes to the ASTMangler in getDeclType
which causes getDeclType to set a flag that says that we have not yet recursed
through the system and thus should suppress the printing of sendable. Once we
get further into the system and recurse, that flag is by default set to true, so
we get the old sending parameter without having to update large amounts of code.
rdar://127383107
A few things:
1. Internally except for in the parser and the clang importer, we only represent
'sending'. This means that it will be easy to remove 'transferring' once enough
time has passed.
2. I included a warning that suggested to the user to change 'transferring' ->
'sending'.
3. I duplicated the parsing diagnostics for 'sending' so both will still get
different sets of diagnostics for parsing issues... but anywhere below parsing,
I have just changed 'transferring' to 'sending' since transferring isn't
represented at those lower levels.
4. Since SendingArgsAndResults is always enabled when TransferringArgsAndResults
is enabled (NOTE not vis-a-versa), we know that we can always parse sending. So
we import "transferring" as "sending". This means that even if one marks a
function with "transferring", the compiler will guard it behind a
SendingArgsAndResults -D flag and in the imported header print out sending.
rdar://128216574
