[NFC] Add a Builder.insertionBlock API.

Some utilities, like Builder.insert(after:) pass a Builder object that
represents an insertion point. That insertion point is sometimes needed to
determine which instructions to create (endApply vs. abortApply). But there is
no way to recover the insertion point. We don't want to simply return an
instruction, because that could be interpreted in different ways. Instead we
provide insertion block, but only in those situations where it makes sense and can't
be used incorrectly.

Author: atrick

SwiftCompilerSources/Sources/SIL/Builder.swift

@@ -29,6 +29,18 @@ public struct Builder {
   private let notificationHandler: BridgedChangeNotificationHandler
   private let notifyNewInstruction: (Instruction) -> ()
 
+  /// Return 'nil' when inserting at the start of a function or in a global initializer.
+  public var insertionBlock: BasicBlock? {
+    switch insertAt {
+    case let .before(inst):
+      return inst.parentBlock
+    case let .atEndOf(block):
+      return block
+    case .atStartOf, .staticInitializer:
+      return nil
+    }
+  }
+
   public var bridged: BridgedBuilder {
     switch insertAt {
     case .before(let inst):