Swift SIL: mark SIL object classes as immortal

This avoids ARC operations on those objects which improves compile time.

Author: eeckstein

SwiftCompilerSources/Sources/SIL/BasicBlock.swift

@@ -13,6 +13,7 @@
 import Basic
 import SILBridging
 
+@_semantics("arc.immortal")
 final public class BasicBlock : CustomStringConvertible, HasShortDescription {
   public var next: BasicBlock? { SILBasicBlock_next(bridged).block }
   public var previous: BasicBlock? { SILBasicBlock_previous(bridged).block }

SwiftCompilerSources/Sources/SIL/Function.swift

@@ -13,6 +13,7 @@
 import Basic
 import SILBridging
 
+@_semantics("arc.immortal")
 final public class Function : CustomStringConvertible, HasShortDescription, Hashable {
   public private(set) var effects = FunctionEffects()
 

SwiftCompilerSources/Sources/SIL/Instruction.swift

@@ -17,6 +17,7 @@ import SILBridging
 //                       Instruction base classes
 //===----------------------------------------------------------------------===//
 
+@_semantics("arc.immortal")
 public class Instruction : CustomStringConvertible, Hashable {
   final public var next: Instruction? {
     SILInstruction_next(bridged).instruction

SwiftCompilerSources/Sources/SIL/Value.swift

@@ -13,6 +13,7 @@
 import Basic
 import SILBridging
 
+@_semantics("arc.immortal")
 public protocol Value : AnyObject, CustomStringConvertible {
   var uses: UseList { get }
   var type: Type { get }