[clang-format] Allow trailing return types in macros

The trailing return type arrow checker verifies that a declaration is
being parsed, however, this isn't true when inside of macros.

It turns out the existence of the auto keyword is enough to make
sure that we're dealing with a trailing return type, and whether we're
in a declaration doesn't matter.

Fixes #47664

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D141811

clang/lib/Format/TokenAnnotator.cpp

@@ -1909,7 +1909,8 @@ class AnnotatingParser {
     } else if (Current.is(tok::arrow) &&
                Style.Language == FormatStyle::LK_Java) {
       Current.setType(TT_LambdaArrow);
-    } else if (Current.is(tok::arrow) && AutoFound && Line.MustBeDeclaration &&
+    } else if (Current.is(tok::arrow) && AutoFound &&
+               (Line.MustBeDeclaration || Line.InPPDirective) &&
                Current.NestingLevel == 0 &&
                !Current.Previous->isOneOf(tok::kw_operator, tok::identifier)) {
       // not auto operator->() -> xxx;

clang/unittests/Format/FormatTest.cpp

@@ -8010,6 +8010,11 @@ TEST_F(FormatTest, TrailingReturnType) {
                "auto aaaaaaaaaaaaaaaaaaaaaa(T t)\n"
                "    -> decltype(eaaaaaaaaaaaaaaa<T>(t.a).aaaaaaaa());");
 
+  FormatStyle Style = getLLVMStyleWithColumns(60);
+  verifyFormat("#define MAKE_DEF(NAME)                                     \\\n"
+               "  auto NAME() -> int { return 42; }",
+               Style);
+
   // Not trailing return types.
   verifyFormat("void f() { auto a = b->c(); }");
   verifyFormat("auto a = p->foo();");