[Options] Switch -Isystem to -I if the front end doesn't support -Isystem

If the front end doesn't support -Isystem, pass it as -I instead. rdar://152602098

Author: ian-twilightcoder

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -190,7 +190,17 @@ extension Driver {
  }
 
  // TODO: Can we drop all search paths for compile jobs for explicit module build?
- try addAllArgumentsWithPath(.I, .Isystem, to: &commandLine, remap: jobNeedPathRemap)
+ if isFrontendArgSupported(.Isystem) {
+ try addAllArgumentsWithPath(.I, .Isystem, to: &commandLine, remap: jobNeedPathRemap)
+ } else {
+ for matching in parsedOptions.arguments(for: .I, .Isystem) {
+ var parsedOption = matching
+ if parsedOption.option == .Isystem {
+ parsedOption = ParsedOption(option: .I, argument: parsedOption.argument, index: parsedOption.index)
+ }
+ try addPathOption(parsedOption, to: &commandLine, remap: jobNeedPathRemap)
+ }
+ }
  try addAllArgumentsWithPath(.F, .Fsystem, to: &commandLine, remap: jobNeedPathRemap)
  try addAllArgumentsWithPath(.vfsoverlay, to: &commandLine, remap: jobNeedPathRemap)
 

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -229,10 +229,11 @@ final class SwiftDriverTests: XCTestCase {
  // The relative ordering of -F and -Fsystem options should be preserved.
  // The relative ordering of -I and -Isystem, and -F and -Fsystem options should be preserved,
  // but all -I options should come before all -F options.
+ let IsystemFlag = driver.isFrontendArgSupported(.Isystem) ? "-Isystem" : "-I"
  try XCTAssertJobInvocationMatches(jobs[0],
  .flag("-I"),
  .path(.absolute(.init(validating: "/path/to/modules"))),
- .flag("-Isystem"),
+ .flag(IsystemFlag),
  .path(.absolute(.init(validating: "/path/to/systemmodules"))),
  .flag("-I"),
  .path(.absolute(.init(validating: "/path/to/more/modules"))),