[Lexer] Clear trivia at the top of lexImpl()

To make sure we only parse trivia for the current token.

Author: rintaro

include/swift/Parse/Lexer.h

@@ -233,11 +233,8 @@ class Lexer {
     Result = NextToken;
     LeadingTriviaResult = {LeadingTrivia};
     TrailingTriviaResult = {TrailingTrivia};
-    if (Result.isNot(tok::eof)) {
-      LeadingTrivia.clear();
-      TrailingTrivia.clear();
+    if (Result.isNot(tok::eof))
       lexImpl();
-    }
   }
 
   void lex(Token &Result) {
@@ -288,8 +285,6 @@ class Lexer {
   void restoreState(State S, bool enableDiagnostics = false) {
     assert(S.isValid());
     CurPtr = getBufferPtrForSourceLoc(S.Loc);
-    LeadingTrivia.clear();
-    TrailingTrivia.clear();
     // Don't reemit diagnostics while readvancing the lexer.
     llvm::SaveAndRestore<DiagnosticEngine*>
       D(Diags, enableDiagnostics ? Diags : nullptr);

lib/Parse/Lexer.cpp

@@ -2058,6 +2058,10 @@ void Lexer::lexImpl() {
   assert(CurPtr >= BufferStart &&
          CurPtr <= BufferEnd && "Current pointer out of range!");
 
+  if (TriviaRetention == TriviaRetentionMode::WithTrivia) {
+    LeadingTrivia.clear();
+    TrailingTrivia.clear();
+  }
   NextToken.setAtStartOfLine(CurPtr == BufferStart);
 
   // Remember where we started so that we can find the comment range.