clang-format: [JS] do not merge side-effect imports.

[mprobst]

Summary

The if condition was testing the current element, but
forgot to check the previous element (doh), so it
would fail depending on sort order of the imports.

clang/lib/Format/SortJavaScriptImports.cpp

void mergeModuleReferences(SmallVector<JsModuleReference, 16> &References,
     while (It != std::end(Indices)) {
       JsModuleReference *Reference = &References[*It];
       // Skip:
       //   import 'foo';
       //   import * as foo from 'foo'; on either previous or this.
       //   import Default from 'foo'; on either previous or this.
       //   mismatching
       if (Reference->Category == JsModuleReference::SIDE_EFFECT ||
+          PreviousReference->Category == JsModuleReference::SIDE_EFFECT ||
           Reference->IsExport != PreviousReference->IsExport ||
           !PreviousReference->Prefix.empty() || !Reference->Prefix.empty() ||
           !PreviousReference->DefaultImport.empty() ||
           !Reference->DefaultImport.empty() || Reference->Symbols.empty() ||
           PreviousReference->URL != Reference->URL) {
         PreviousReference = Reference;
         ++It;
         continue;

clang/unittests/Format/SortImportsTestJS.cpp

TEST_F(SortImportsTestJS, MergeImports) {
   // do not merge imports and exports
   verifySort("import {A} from 'foo';\n"
              "export {B} from 'foo';",
              "import {A} from 'foo';\n"
              "export   {B} from 'foo';");
   // do merge exports
   verifySort("export {A, B} from 'foo';\n", "export {A} from 'foo';\n"
                                             "export   {B} from 'foo';");
+
+  // do not merge side effect imports with named ones
+  verifySort("import './a';\n"
+             "\n"
+             "import {bar} from './a';\n",
+             "import {bar} from './a';\n"
+             "import './a';\n");
 }
 
 } // end namespace
 } // end namespace format
 } // end namespace clang