[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 +190,16 @@ 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) {
+        commandLine.appendFlag(.I)
+        let path = try VirtualPath(path: matching.argument.asSingle)
+        let needRemap = jobNeedPathRemap && isCachingEnabled
+        commandLine.appendPath(needRemap ? remapPath(path) : path)
+      }
+    }
     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"))),