[Options] -Isystem doesn't work everywhere -I does

Pass -Isystem along everywhere that -I is.

rdar://152331718

Author: ian-twilightcoder

Sources/SwiftDriver/Jobs/APIDigesterJobs.swift

@@ -194,6 +194,10 @@ extension Driver {
     commandLine.appendPath(VirtualPath.lookup(frontendTargetInfo.runtimeResourcePath.path))
 
     try commandLine.appendAll(.I, from: &parsedOptions)
+    for systemImport in parsedOptions.arguments(for: .Isystem) {
+      commandLine.appendFlag(.isystem)
+      commandLine.appendFlag(systemImport.argument.asSingle)
+    }
     try commandLine.appendAll(.F, from: &parsedOptions)
     for systemFramework in parsedOptions.arguments(for: .Fsystem) {
       commandLine.appendFlag(.iframework)

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -190,7 +190,7 @@ extension Driver {
     }
 
     // TODO: Can we drop all search paths for compile jobs for explicit module build?
-    try addAllArgumentsWithPath(.I, to: &commandLine, remap: jobNeedPathRemap)
+    try addAllArgumentsWithPath(.I, .Isystem, to: &commandLine, remap: jobNeedPathRemap)
     try addAllArgumentsWithPath(.F, .Fsystem, to: &commandLine, remap: jobNeedPathRemap)
     try addAllArgumentsWithPath(.vfsoverlay, to: &commandLine, remap: jobNeedPathRemap)
 

Sources/SwiftOptions/Options.swift

@@ -601,6 +601,7 @@ extension Option {
   public static let h: Option = Option("-h", .flag, alias: Option.help)
   public static let IEQ: Option = Option("-I=", .joined, alias: Option.I, attributes: [.frontend, .argumentIsPath])
   public static let iframework: Option = Option("-iframework", .joinedOrSeparate, attributes: [.noDriver, .argumentIsPath], helpText: "add a directory to the clang importer system framework search path")
+  public static let isystem: Option = Option("-isystem", .joinedOrSeparate, attributes: [.noDriver, .argumentIsPath], helpText: "add a directory to the clang importer system header search path")
   public static let ignoreAlwaysInline: Option = Option("-ignore-always-inline", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Ignore @inline(__always) attributes.")
   public static let ignoreModuleSourceInfo: Option = Option("-ignore-module-source-info", .flag, attributes: [.frontend, .noDriver], helpText: "Avoid getting source location from .swiftsourceinfo files")
   public static let ignoreSpiGroups: Option = Option("-ignore-spi-group", .separate, attributes: [.noDriver], helpText: "SPI group name to not diagnose about")

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -219,12 +219,23 @@ final class SwiftDriverTests: XCTestCase {
   func testRelativeOptionOrdering() throws {
     var driver = try Driver(args: ["swiftc", "foo.swift",
                                    "-F", "/path/to/frameworks",
+                                   "-I", "/path/to/modules",
                                    "-Fsystem", "/path/to/systemframeworks",
-                                   "-F", "/path/to/more/frameworks"])
+                                   "-Isystem", "/path/to/systemmodules",
+                                   "-F", "/path/to/more/frameworks",
+                                   "-I", "/path/to/more/modules"])
     let jobs = try driver.planBuild()
     XCTAssertEqual(jobs[0].kind, .compile)
     // 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.
     try XCTAssertJobInvocationMatches(jobs[0],
+                                     .flag("-I"),
+                                     .path(.absolute(.init(validating: "/path/to/modules"))),
+                                     .flag("-Isystem"),
+                                     .path(.absolute(.init(validating: "/path/to/systemmodules"))),
+                                     .flag("-I"),
+                                     .path(.absolute(.init(validating: "/path/to/more/modules"))),
                                      .flag("-F"),
                                      .path(.absolute(.init(validating: "/path/to/frameworks"))),
                                      .flag("-Fsystem"),