[Integrated Swift driver] Pass along the Swift location.

Rather than letting the integrated Swift driver guess at where the
Swift frontend is located, pass through the location of the Swift
compiler. This also allows SWIFT_EXEC overrides to work with the
integrated driver.

Author: DougGregor

Sources/Build/ManifestBuilder.swift

@@ -200,29 +200,28 @@ extension LLBuildManifestBuilder {
         do {
             // Use the integrated Swift driver to compute the set of frontend
             // jobs needed to build this Swift target.
-            var driver = try Driver(args: target.emitCommandLine(), fileSystem: target.fs)
+            var commandLine = target.emitCommandLine();
+            commandLine.append("-driver-use-frontend-path")
+            commandLine.append(buildParameters.toolchain.swiftCompiler.pathString)
+            var driver = try Driver(args: commandLine, fileSystem: target.fs)
             let jobs = try driver.planBuild()
             let resolver = try ArgsResolver(fileSystem: target.fs)
 
             for job in jobs {
-                // Figure out which tool we are using.
-                // FIXME: This feels like a hack.
-                let datool: String
+                let tool = try resolver.resolve(.path(job.tool))
                 let isSwiftFrontend: Bool
                 switch job.kind {
                 case .compile, .mergeModule, .emitModule, .generatePCH,
                     .generatePCM, .interpret, .repl, .printTargetInfo,
                     .versionRequest, .backend:
-                    datool = buildParameters.toolchain.swiftCompiler.pathString
                     isSwiftFrontend = true
 
                 case .autolinkExtract, .generateDSYM, .help, .link, .verifyDebugInfo:
-                    datool = try resolver.resolve(.path(job.tool))
                     isSwiftFrontend = false
                 }
 
                 let commandLine = try job.commandLine.map{ try resolver.resolve($0) }
-                let arguments = [datool] + commandLine
+                let arguments = [tool] + commandLine
 
                 let jobInputs = job.inputs.map { $0.resolveToNode() }
                 let jobOutputs = job.outputs.map { $0.resolveToNode() }