[Driver] Handle outstanding jobs by reading the build record.

r23968 wrote out a record of which source files were included in a build,
and whether they were succesfully compiled or not...and if not, whether
they were out of date because of a cascading or non-cascading dependency.
This commit uses that information to decide what files might need to be
rebuilt even if a particular input doesn't change and doesn't appear to
have any changed dependencies. The two interesting cases are:

- A file was going to be built last time, but the build was halted
  because of an error. Build it this time.
- One of the files was removed and thus we've lost a source of dependency
  information; rebuild everything!

rdar://problem/19270980

Swift SVN r24018

lib/Driver/Compilation.cpp

@@ -158,9 +158,11 @@ int Compilation::performJobsInList(const JobList &JL, PerformJobsState &State) {
 
     switch (Condition) {
     case Job::Condition::Always:
-      scheduleCommandIfNecessaryAndPossible(Cmd);
       if (getIncrementalBuildEnabled() && !DependenciesFile.empty())
         InitialOutOfDateCommands.push_back(Cmd);
+      SWIFT_FALLTHROUGH;
+    case Job::Condition::RunWithoutCascading:
+      scheduleCommandIfNecessaryAndPossible(Cmd);
       break;
     case Job::Condition::CheckDependencies:
       DeferredCommands.insert(Cmd);
@@ -332,16 +334,13 @@ int Compilation::performJobsInList(const JobList &JL, PerformJobsState &State) {
     assert(State.BlockingCommands.empty() &&
            "some blocking commands never finished properly");
   } else {
-    // Make sure we record any files that haven't been touched but nonetheless
-    // need to be rebuilt.
+    // Make sure we record any files that still need to be rebuilt.
     for (const Job *Cmd : JL) {
-      switch (Cmd->getCondition()) {
-      case Job::Condition::Always:
-        // Don't worry about files that would be rebuilt anyway.
+      // Skip files that don't use dependency analysis.
+      StringRef DependenciesFile =
+          Cmd->getOutput().getAdditionalOutputForType(types::TY_SwiftDeps);
+      if (DependenciesFile.empty())
         continue;
-      case Job::Condition::CheckDependencies:
-        break;
-      }
 
       // Don't worry about commands that finished or weren't going to run.
       if (State.FinishedCommands.count(Cmd))
@@ -349,6 +348,7 @@ int Compilation::performJobsInList(const JobList &JL, PerformJobsState &State) {
       if (!State.ScheduledCommands.count(Cmd))
         continue;
 
+
       State.UnfinishedCommands.insert({Cmd, DepGraph.isMarked(Cmd)});
     }
   }
@@ -373,6 +373,7 @@ int Compilation::performSingleCommand(const Job *Cmd) {
   switch (Cmd->getCondition()) {
   case Job::Condition::CheckDependencies:
     return 0;
+  case Job::Condition::RunWithoutCascading:
   case Job::Condition::Always:
     break;
   }