clang-format: [JS] consistenly format enums.

[mprobst]

Summary

Previously, const enums would get formatted differently because the modifier was not recognized.

[djasper]

LG

lib/Format/TokenAnnotator.cpp

if (Left.is(TT_DictLiteral) && Left.is(tok::l_brace) && Line.Level == 0 &&
         // kw_var/kw_let are pseudo-tokens that are tok::identifier, so match
         // above.
         !Line.First->isOneOf(Keywords.kw_var, Keywords.kw_let))
       // Object literals on the top level of a file are treated as "enum-style".
       // Each key/value pair is put on a separate line, instead of bin-packing.
       return true;
     if (Left.is(tok::l_brace) && Line.Level == 0 &&
         (Line.startsWith(tok::kw_enum) ||
-         Line.startsWith(tok::kw_export, tok::kw_enum)))
+         Line.startsWith(tok::kw_const, tok::kw_enum) ||
+         Line.startsWith(tok::kw_export, tok::kw_enum) ||
+         Line.startsWith(tok::kw_export, tok::kw_const, tok::kw_enum)))
       // JavaScript top-level enum key/value pairs are put on separate lines
       // instead of bin-packing.
       return true;
     if (Right.is(tok::r_brace) && Left.is(tok::l_brace) &&
         !Left.Children.empty())
       // Support AllowShortFunctionsOnASingleLine for JavaScript.
       return Style.AllowShortFunctionsOnASingleLine == FormatStyle::SFS_None ||
              Style.AllowShortFunctionsOnASingleLine == FormatStyle::SFS_Empty ||

unittests/Format/FormatTestJS.cpp

verifyFormat("export /* somecomment*/ enum Foo {\n"
                "  A = 1,\n"
                "  B\n"
                "}");
   verifyFormat("enum Foo {\n"
                "  A = 1,  // comment\n"
                "  B\n"
                "}\n"
                "var x = 1;");
+  verifyFormat("const enum Foo {\n"
+               "  A = 1,\n"
+               "  B\n"
+               "}");
+  verifyFormat("export const enum Foo {\n"
+               "  A = 1,\n"
+               "  B\n"
+               "}");
 }
 
 TEST_F(FormatTestJS, MetadataAnnotations) {
   verifyFormat("@A\nclass C {\n}");
   verifyFormat("@A({arg: 'value'})\nclass C {\n}");
   verifyFormat("@A\n@B\nclass C {\n}");
   verifyFormat("class C {\n  @A x: string;\n}");
   verifyFormat("class C {\n"