Make with*Continuation inherit the caller's executor.

Not inheriting the caller's executor is a major problem for
these functions.  Under SE-0338, treating them as non-isolated
means that it's illegal to pass them anytthing non-Sendable
from a different isolation context; since the function is
declared to take a non-sendable function parameter, effectively,
SE-0338 means that these functions can only be safely called
from non-isolated contexts.  That's not really acceptable, but
it gets worse: since we haven't implemented the sendability
rule for that yet, we're immediately bypassing isolation safety
when using these functions with no warning.

The `withCheckedContinuation` functions are not
`@_alwaysEmitIntoClient` (an oversight in the initial release),
and so we need to use the unsafe attribute on them.  When we
eventually implement a safe mechanism for this, we should make
`@_alwaysEmitIntoClient` variants of these functions which use
the new feature, and we can demote the existing functions to
`internal @availableFromInline`.

I've gone ahead and made `withChecked*Continuation` `@inlinable`.

Author: rjmccall

stdlib/public/Concurrency/CheckedContinuation.swift

@@ -265,6 +265,8 @@ extension CheckedContinuation {
 ///   - body: A closure that takes a `CheckedContinuation` parameter.
 ///     You must resume the continuation exactly once.
 @available(SwiftStdlib 5.1, *)
+@_unsafeInheritExecutor // ABI compatibility with Swift 5.1
+@inlinable
 public func withCheckedContinuation<T>(
     function: String = #function,
     _ body: (CheckedContinuation<T, Never>) -> Void
@@ -287,6 +289,8 @@ public func withCheckedContinuation<T>(
 /// If `resume(throwing:)` is called on the continuation,
 /// this function throws that error.
 @available(SwiftStdlib 5.1, *)
+@_unsafeInheritExecutor // ABI compatibility with Swift 5.1
+@inlinable
 public func withCheckedThrowingContinuation<T>(
     function: String = #function,
     _ body: (CheckedContinuation<T, Error>) -> Void

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -257,6 +257,7 @@ internal func _resumeUnsafeThrowingContinuationWithError<T>(
 ///
 /// - Returns: The value passed to the continuation by the closure.
 @available(SwiftStdlib 5.1, *)
+@_unsafeInheritExecutor
 @_alwaysEmitIntoClient
 public func withUnsafeContinuation<T>(
   _ fn: (UnsafeContinuation<T, Never>) -> Void
@@ -277,6 +278,7 @@ public func withUnsafeContinuation<T>(
 /// If `resume(throwing:)` is called on the continuation,
 /// this function throws that error.
 @available(SwiftStdlib 5.1, *)
+@_unsafeInheritExecutor
 @_alwaysEmitIntoClient
 public func withUnsafeThrowingContinuation<T>(
   _ fn: (UnsafeContinuation<T, Error>) -> Void