[Driver] Always write a build record, even if there's only one input.

Don't just exec() a subprocess if there's more work to do when the build
finishes.

Swift SVN r28775

lib/Driver/Compilation.cpp

@@ -468,9 +468,10 @@ static void writeCompilationRecord(
 }
 
 int Compilation::performJobs() {
-  // We require buffered output if Parseable output was requested.
-  bool RequiresBufferedOutput = (Level == OutputLevel::Parseable);
-  if (!RequiresBufferedOutput) {
+  // If we don't have to do any cleanup work, just exec the subprocess.
+  if (Level < OutputLevel::Parseable &&
+      (SaveTemps || TempFilePaths.empty()) &&
+      CompilationRecordPath.empty()) {
     if (const Job *OnlyCmd = getOnlyCommandInList(Jobs.get()))
       return performSingleCommand(OnlyCmd);
   }

test/Driver/Dependencies/independent.swift

@@ -4,6 +4,7 @@
 // RUN: touch -t 201401240005 %t/*
 
 // RUN: cd %t && %swiftc_driver -c -driver-use-frontend-path %S/Inputs/update-dependencies.py -output-file-map %t/output.json -incremental ./main.swift -j1 -v 2>&1 | FileCheck -check-prefix=CHECK-FIRST %s
+// RUN: ls %t/main~buildrecord.swiftdeps
 
 // CHECK-FIRST-NOT: warning
 // CHECK-FIRST: Handled main.swift
@@ -35,3 +36,12 @@
 // RUN: rm %t/*.o
 // RUN: cd %t && %swiftc_driver -c -driver-use-frontend-path %S/Inputs/update-dependencies.py -output-file-map %t/output.json -incremental ./main.swift ./other.swift -module-name main -j1 -v 2>&1 | FileCheck -check-prefix=CHECK-FIRST-MULTI %s
 
+
+// RUN: rm -rf %t && cp -r %S/Inputs/independent/ %t
+// RUN: touch -t 201401240005 %t/*
+
+// RUN: cd %t && %swiftc_driver -c -driver-use-frontend-path %S/Inputs/update-dependencies.py -output-file-map %t/output.json -incremental ./main.swift -module-name main -j1 -v 2>&1 | FileCheck -check-prefix=CHECK-SINGLE %s
+// CHECK-SINGLE: Handled main.swift
+
+// RUN: ls %t/main~buildrecord.swiftdeps
+