clang-format: [JS] do not merge imports and exports.

[mprobst]

Summary

Previously, clang-format would erroneously merge import and export
statements. These need to be kept separate, as the semantics differ.

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 ||
+          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

verifySort("// a\n"
              "// a\n"
              "import {/* y */ Y} from 'a';\n"
              "// z\n"
              "import {Z} from 'z';\n"
              "// also a\n"
              "import {/* x */ X} from 'a';\n"
              "\n"
              "X + Y + Z;\n");
+
+  // 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';");
 }
 
 } // end namespace
 } // end namespace format
 } // end namespace clang