Keep old build_time field label

Author: David Ungar

Sources/SwiftDriver/IncrementalCompilation/BuildRecord.swift

@@ -41,9 +41,8 @@ public struct BuildRecord {
   private enum SectionName: String, CaseIterable {
     case swiftVersion = "version"
     case argsHash = "options"
-    // Implement this for a smoother transition
-    case legacyBuildStartTime = "build_time"
-    case buildStartTime = "build_start_time"
+    // Keep "build_time" for smoother upgrades/downgrades
+    case buildStartTime = "build_time"
     case buildEndTime = "build_end_time"
     case inputInfos = "inputs"
 
@@ -89,8 +88,7 @@ public extension BuildRecord {
           return nil
         }
         argsHash = s
-      case SectionName.buildStartTime.serializedName,
-           SectionName.legacyBuildStartTime.serializedName:
+      case SectionName.buildStartTime.serializedName:
         guard let d = Self.decodeDate(value,
                                       forInputInfo: false,
                                       failedToReadOutOfDateMap)