[BatchMode] Address review comments.

Author: graydon

include/swift/Driver/Driver.h

@@ -64,7 +64,17 @@ class OutputInfo {
     /// A compilation using a single frontend invocation without -primary-file.
     SingleCompile,
 
-    /// A single process that batches together multiple StandardCompile jobs.
+    /// A single process that batches together multiple StandardCompile Jobs.
+    ///
+    /// Note: this is a transient value to use _only_ for the individual
+    /// BatchJobs that are the temporary containers for multiple StandardCompile
+    /// Jobs built by ToolChain::constructBatchJob.
+    ///
+    /// In particular, the driver treats a batch-mode-enabled Compilation as
+    /// having OutputInfo::CompilerMode == StandardCompile, with the
+    /// Compilation::BatchModeEnabled flag set to true, _not_ as a
+    /// BatchModeCompile Compilation. The top-level OutputInfo::CompilerMode for
+    /// a Compilation should never be BatchModeCompile.
     BatchModeCompile,
 
     /// Invoke the REPL

lib/Driver/Driver.cpp

@@ -1403,19 +1403,18 @@ Driver::computeCompilerMode(const DerivedArgList &Args,
                               options::OPT_disable_batch_mode,
                               false);
 
-  if (ArgRequiringSingleCompile) {
-    // Override batch mode if given -wmo or -index-file.
-    if (BatchModeOut) {
-      BatchModeOut = false;
-      // Emit a warning about such overriding (FIXME: we might conditionalize
-      // this based on the user or xcode passing -disable-batch-mode).
-      Diags.diagnose(SourceLoc(), diag::warn_ignoring_batch_mode,
-                     ArgRequiringSingleCompile->getOption().getPrefixedName());
-    }
-    return OutputInfo::Mode::SingleCompile;
-  }
-
-  return OutputInfo::Mode::StandardCompile;
+  if (!ArgRequiringSingleCompile)
+    return OutputInfo::Mode::StandardCompile;
+
+  // Override batch mode if given -wmo or -index-file.
+  if (BatchModeOut) {
+    BatchModeOut = false;
+    // Emit a warning about such overriding (FIXME: we might conditionalize
+    // this based on the user or xcode passing -disable-batch-mode).
+    Diags.diagnose(SourceLoc(), diag::warn_ignoring_batch_mode,
+                   ArgRequiringSingleCompile->getOption().getPrefixedName());
+  }
+  return OutputInfo::Mode::SingleCompile;
 }
 
 void Driver::buildActions(SmallVectorImpl<const Action *> &TopLevelActions,