clang-format: [JS] fix `??` opreator wrapping.

[mprobst]

Summary

clang-format currently treats the nullish coalescing operator ?? like
the ternary operator. That causes multiple nullish terms to be each
indented relative to the last ??, as they would in a ternary.

The ?? operator is often used in chains though, and as such more
similar to other binary operators, such as ||. So to fix the indent,
set its token type to ||, so it inherits the same treatment.

This opens up the question of operator precedence. However, ?? is
required to be parenthesized when mixed with || and &&, so this is
not a problem that can come up in syntactically legal code.

clang/lib/Format/FormatTokenLexer.cpp

if (Style.Language == FormatStyle::LK_JavaScript) {
     if (tryMergeTokens(JSRightArrow, TT_JsFatArrow))
       return;
     if (tryMergeTokens(JSExponentiation, TT_JsExponentiation))
       return;
     if (tryMergeTokens(JSExponentiationEqual, TT_JsExponentiationEqual)) {
       Tokens.back()->Tok.setKind(tok::starequal);
       return;
     }
-    if (tryMergeTokens(JSNullishOperator, TT_JsNullishCoalescingOperator))
+    if (tryMergeTokens(JSNullishOperator, TT_JsNullishCoalescingOperator)) {
+      // Treat like the "||" operator (as opposed to the ternary ?).
+      Tokens.back()->Tok.setKind(tok::pipepipe);
       return;
+    }
     if (tryMergeTokens(JSNullPropagatingOperator,
                        TT_JsNullPropagatingOperator)) {
       // Treat like a regular "." access.
       Tokens.back()->Tok.setKind(tok::period);
       return;
     }
     if (tryMergeJSPrivateIdentifier())
       return;

clang/unittests/Format/FormatTestJS.cpp

 TEST_F(FormatTestJS, NullPropagatingOperator) {
   verifyFormat("let x = foo?.bar?.baz();\n");
   verifyFormat("let x = foo?.(foo);\n");
   verifyFormat("let x = foo?.['arr'];\n");
 }
 
 TEST_F(FormatTestJS, NullishCoalescingOperator) {
   verifyFormat("const val = something ?? 'some other default';\n");
+  verifyFormat(
+      "const val = something ?? otherDefault ??\n"
+      "    evenMore ?? evenMore;\n",
+      "const val = something ?? otherDefault ?? evenMore ?? evenMore;\n",
+      getGoogleJSStyleWithColumns(40));
 }
 
 TEST_F(FormatTestJS, Conditional) {
   verifyFormat("y = x ? 1 : 2;");
   verifyFormat("x ? 1 : 2;");
   verifyFormat("class Foo {\n"
                "  field = true ? 1 : 2;\n"
                "  method(a = true ? 1 : 2) {}\n"