[stdlib] Generalize temporary allocations for noncopyable result types

We can’t do typed throws here yet. (They involve implicit stack allocations that interfere with the explicit stack allocation builtins.)

(cherry picked from commit 5679bf6)

Author: lorentey

stdlib/public/core/TemporaryAllocation.swift

@@ -124,8 +124,9 @@ internal func _isStackAllocationSafe(byteCount: Int, alignment: Int) -> Bool {
 /// This function encapsulates the various calls to builtins required by
 /// `withUnsafeTemporaryAllocation()`.
 @_alwaysEmitIntoClient @_transparent
-// FIXME(NCG): R needs to be ~Copyable too, but that leads to lifetime failures (rdar://124571365).
-internal func _withUnsafeTemporaryAllocation<T: ~Copyable, R>(
+internal func _withUnsafeTemporaryAllocation<
+  T: ~Copyable, R: ~Copyable
+>(
   of type: T.Type,
   capacity: Int,
   alignment: Int,
@@ -169,8 +170,7 @@ internal func _withUnsafeTemporaryAllocation<T: ~Copyable, R>(
 
 @_alwaysEmitIntoClient @_transparent
 internal func _withUnprotectedUnsafeTemporaryAllocation<
-  // FIXME(NCG): R needs to be ~Copyable too, but that leads to lifetime failures (rdar://124571365).
-  T: ~Copyable, R
+  T: ~Copyable, R: ~Copyable
 >(
   of type: T.Type,
   capacity: Int,
@@ -267,8 +267,7 @@ internal func _fallBackToHeapAllocation<R: ~Copyable>(
 /// the buffer) must not escape. It will be deallocated when `body` returns and
 /// cannot be used afterward.
 @_alwaysEmitIntoClient @_transparent
-// FIXME(NCG): R needs to be ~Copyable, but that leads to lifetime failures (rdar://124571365).
-public func withUnsafeTemporaryAllocation<R>(
+public func withUnsafeTemporaryAllocation<R: ~Copyable>(
   byteCount: Int,
   alignment: Int,
   _ body: (UnsafeMutableRawBufferPointer) throws -> R
@@ -292,8 +291,7 @@ public func withUnsafeTemporaryAllocation<R>(
 /// This function is similar to `withUnsafeTemporaryAllocation`, except that it
 /// doesn't trigger stack protection for the stack allocated memory.
 @_alwaysEmitIntoClient @_transparent
-// FIXME(NCG): R needs to be ~Copyable, but that leads to lifetime failures (rdar://124571365).
-public func _withUnprotectedUnsafeTemporaryAllocation<R>(
+public func _withUnprotectedUnsafeTemporaryAllocation<R: ~Copyable>(
   byteCount: Int,
   alignment: Int,
   _ body: (UnsafeMutableRawBufferPointer) throws -> R
@@ -343,8 +341,9 @@ public func _withUnprotectedUnsafeTemporaryAllocation<R>(
 /// the buffer) must not escape. It will be deallocated when `body` returns and
 /// cannot be used afterward.
 @_alwaysEmitIntoClient @_transparent
-// FIXME(NCG): R needs to be ~Copyable too, but that leads to lifetime failures (rdar://124571365).
-public func withUnsafeTemporaryAllocation<T: ~Copyable, R>(
+public func withUnsafeTemporaryAllocation<
+  T: ~Copyable,R: ~Copyable
+>(
   of type: T.Type,
   capacity: Int,
   _ body: (UnsafeMutableBufferPointer<T>) throws -> R
@@ -369,8 +368,9 @@ public func withUnsafeTemporaryAllocation<T: ~Copyable, R>(
 /// This function is similar to `withUnsafeTemporaryAllocation`, except that it
 /// doesn't trigger stack protection for the stack allocated memory.
 @_alwaysEmitIntoClient @_transparent
-// FIXME(NCG): R needs to be ~Copyable too, but that leads to lifetime failures (rdar://124571365).
-public func _withUnprotectedUnsafeTemporaryAllocation<T: ~Copyable, R>(
+public func _withUnprotectedUnsafeTemporaryAllocation<
+  T: ~Copyable, R: ~Copyable
+>(
   of type: T.Type,
   capacity: Int,
   _ body: (UnsafeMutableBufferPointer<T>) throws -> R