Add support for macro frontend options

Pass through options for macros, especially compiler plugin search
paths. Add a default search path into `lib/swift/host/plugins`.

Driver side of rdar://105095761

Author: DougGregor

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -237,6 +237,19 @@ extension Driver {
       commandLine.appendFlag(.emptyAbiDescriptor)
     }
 
+    if isFrontendArgSupported(.emitMacroExpansionFiles) {
+      try commandLine.appendLast(.emitMacroExpansionFiles, from: &parsedOptions)
+    }
+
+    if isFrontendArgSupported(.pluginPath) {
+      try commandLine.appendAll(.pluginPath, from: &parsedOptions)
+
+      let defaultPluginPath = try toolchain.executableDir.parentDirectory
+        .appending(components: "lib", "swift", "host", "plugins")
+      commandLine.appendFlag(.pluginPath)
+      commandLine.appendPath(defaultPluginPath)
+    }
+
     // Pass down -user-module-version if we are working with a compiler that
     // supports it.
     if let ver = parsedOptions.getLastArgument(.userModuleVersion)?.asSingle,

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -6633,6 +6633,19 @@ final class SwiftDriverTests: XCTestCase {
 #endif
   }
 
+  func testPluginPaths() throws {
+    var driver = try Driver(args: ["swiftc", "-typecheck", "foo.swift"])
+    guard driver.isFrontendArgSupported(.pluginPath) else {
+      return
+    }
+
+    let jobs = try driver.planBuild().removingAutolinkExtractJobs()
+    XCTAssertEqual(jobs.count, 1)
+    let job = jobs.first!
+    XCTAssertTrue(job.commandLine.contains(.flag("-plugin-path")))
+    XCTAssertTrue(job.commandLine.contains(.path(.absolute(try driver.toolchain.executableDir.parentDirectory.appending(components: "lib", "swift", "host", "plugins")))))
+  }
+
   func testRegistrarLookup() throws {
 #if os(Windows)
     let SDKROOT: AbsolutePath = localFileSystem.currentWorkingDirectory!.appending(components: "SDKROOT")