[clang-format] Refactor common handling of attributes. NFC.

[MyDeveloperDay]

LGTM

clang/lib/Format/UnwrappedLineParser.cpp

do {
     default:
       nextToken();
       break;
     }
   } while (!eof());
 }
 
 void UnwrappedLineParser::parseIfThenElse() {
+  auto HandleAttributes = [this]() {
+    // Handle AttributeMacro, e.g. `if (x) UNLIKELY`.
+    if (FormatTok->is(TT_AttributeMacro))
+      nextToken();
+    // Handle [[likely]] / [[unlikely]] attributes.
+    if (FormatTok->is(tok::l_square) && tryToParseSimpleAttribute())
+      parseSquare();
+  };
+
   assert(FormatTok->Tok.is(tok::kw_if) && "'if' expected");
   nextToken();
   if (FormatTok->Tok.isOneOf(tok::kw_constexpr, tok::identifier))
     nextToken();
   if (FormatTok->Tok.is(tok::l_paren))
     parseParens();
-  // handle  AttributeMacro  if (x) UNLIKELY
-  if (FormatTok->is(TT_AttributeMacro))
-    nextToken();
-  // handle [[likely]] / [[unlikely]]
-  if (FormatTok->is(tok::l_square) && tryToParseSimpleAttribute())
-    parseSquare();
+  HandleAttributes();
   bool NeedsUnwrappedLine = false;
   if (FormatTok->Tok.is(tok::l_brace)) {
     CompoundStatementIndenter Indenter(this, Style, Line->Level);
     parseBlock();
     if (Style.BraceWrapping.BeforeElse)
       addUnwrappedLine();
     else
       NeedsUnwrappedLine = true;
   } else {
     addUnwrappedLine();
     ++Line->Level;
     parseStructuralElement();
     --Line->Level;
   }
   if (FormatTok->Tok.is(tok::kw_else)) {
     nextToken();
-    // handle  AttributeMacro  else UNLIKELY
-    if (FormatTok->is(TT_AttributeMacro))
-      nextToken();
-    // handle [[likely]] / [[unlikely]]
-    if (FormatTok->Tok.is(tok::l_square) && tryToParseSimpleAttribute())
-      parseSquare();
+    HandleAttributes();
     if (FormatTok->Tok.is(tok::l_brace)) {
       CompoundStatementIndenter Indenter(this, Style, Line->Level);
       parseBlock();
       addUnwrappedLine();
     } else if (FormatTok->Tok.is(tok::kw_if)) {
       FormatToken *Previous = Tokens->getPreviousToken();
       bool PrecededByComment = Previous && Previous->is(tok::comment);
       if (PrecededByComment) {