[parser] Add comments that parseStmt() and isStartOfStmt() must be kept in sync, NFC

Author: akyrtzi

lib/Parse/ParseStmt.cpp

@@ -36,6 +36,9 @@ using namespace swift::syntax;
 /// isStartOfStmt - Return true if the current token starts a statement.
 ///
 bool Parser::isStartOfStmt() {
+  // This needs to be kept in sync with `Parser::parseStmt()`. If a new token
+  // kind is accepted here as start of statement, it should also be handled in
+  // `Parser::parseStmt()`.
   switch (Tok.getKind()) {
   default: return false;
   case tok::kw_return:
@@ -588,7 +591,9 @@ ParserResult<Stmt> Parser::parseStmt() {
   if (isContextualYieldKeyword()) {
     Tok.setKind(tok::kw_yield);
   }
-  
+
+  // This needs to handle everything that `Parser::isStartOfStmt()` accepts as
+  // start of statement.
   switch (Tok.getKind()) {
   case tok::pound_line:
   case tok::pound_sourceLocation: