[ABIChecking] Use -Isystem and -Fsystem for swift-api-digester

swift-api-digester supports the Swift -Fsystem and -Isystem flags, switch to those. rdar://152747553

Author: ian-twilightcoder

Sources/SwiftDriver/Jobs/APIDigesterJobs.swift

@@ -193,16 +193,8 @@ extension Driver {
  commandLine.appendFlag(.resourceDir)
  commandLine.appendPath(VirtualPath.lookup(frontendTargetInfo.runtimeResourcePath.path))
 
- try commandLine.appendAll(.I, from: &parsedOptions)
- for systemImport in parsedOptions.arguments(for: .Isystem) {
- commandLine.appendFlag(.I)
- commandLine.appendFlag(systemImport.argument.asSingle)
- }
- try commandLine.appendAll(.F, from: &parsedOptions)
- for systemFramework in parsedOptions.arguments(for: .Fsystem) {
- commandLine.appendFlag(.iframework)
- commandLine.appendFlag(systemFramework.argument.asSingle)
- }
+ try commandLine.appendAll(.I, .Isystem, from: &parsedOptions)
+ try commandLine.appendAll(.F, .Fsystem, from: &parsedOptions)
 
  try commandLine.appendLast(.swiftVersion, from: &parsedOptions)
  }