[Frontend] Attach comments to decls when emitting .swiftsourceinfo

.swiftsourceinfo contains comment ranges, so parsing should attach
comments when it is being emitted. Generally this turned out to be fine
because .swiftdoc is usually output as well, but when calling
swift-frontend directly in tests that isn't necessarily the case.

Author: bnbarham

lib/Frontend/CompilerInvocation.cpp

@@ -704,21 +704,15 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.VerifySyntaxTree = true;
   }
 
+  // Configure lexing to parse and remember comments if:
+  //   - Emitting a swiftdoc/swiftsourceinfo
+  //   - Performing index-while-building
+  //   - Emitting a symbol graph file
   // If we are asked to emit a module documentation file, configure lexing and
   // parsing to remember comments.
-  if (FrontendOpts.InputsAndOutputs.hasModuleDocOutputPath()) {
-    Opts.AttachCommentsToDecls = true;
-  }
-
-  // If we are doing index-while-building, configure lexing and parsing to
-  // remember comments.
-  if (!FrontendOpts.IndexStorePath.empty()) {
-    Opts.AttachCommentsToDecls = true;
-  }
-
-  // If we are emitting a symbol graph file, configure lexing and parsing to
-  // remember comments.
-  if (FrontendOpts.EmitSymbolGraph) {
+  if (FrontendOpts.InputsAndOutputs.hasModuleDocOutputPath() ||
+      FrontendOpts.InputsAndOutputs.hasModuleSourceInfoOutputPath() ||
+      !FrontendOpts.IndexStorePath.empty() || FrontendOpts.EmitSymbolGraph) {
     Opts.AttachCommentsToDecls = true;
   }