[Parse] Update Lexer::getLocFor*Line methods to return correct locations

When the location given to getLocForStartOfLine was an empty line, it
would actually return the location of the next line rather than the
given location as it should.

If the location given to getLocForEndOfLine was inside a token on a line
that was either empty or contained whitespace, it would skip to the end
of that token and then return the location for the next line. This was
an issue for multiline strings, where the string is a single token but
it's over multiple lines.

Author: bnbarham

include/swift/Parse/Lexer.h

@@ -352,7 +352,7 @@ class Lexer {
   static SourceLoc getLocForStartOfLine(SourceManager &SM, SourceLoc Loc);
 
   /// Retrieve the source location for the end of the line containing the
-  /// given token, which is the location of the start of the next line.
+  /// given location, which is the location of the start of the next line.
   static SourceLoc getLocForEndOfLine(SourceManager &SM, SourceLoc Loc);
 
   /// Retrieve the string used to indent the line that contains the given

lib/Parse/Lexer.cpp

@@ -2706,12 +2706,12 @@ static SourceLoc getLocForStartOfTokenInBuf(SourceManager &SM,
 // Find the start of the given line.
 static const char *findStartOfLine(const char *bufStart, const char *current) {
   while (current != bufStart) {
-    if (current[0] == '\n' || current[0] == '\r') {
+    --current;
+
+    if (current[0] == '\n') {
       ++current;
       break;
     }
-
-    --current;
   }
 
   return current;
@@ -2779,19 +2779,16 @@ SourceLoc Lexer::getLocForEndOfLine(SourceManager &SM, SourceLoc Loc) {
   if (BufferID < 0)
     return SourceLoc();
 
-  // Use fake language options; language options only affect validity
-  // and the exact token produced.
-  LangOptions FakeLangOpts;
+  CharSourceRange entireRange = SM.getRangeForBuffer(BufferID);
+  StringRef Buffer = SM.extractText(entireRange);
 
-  // Here we return comments as tokens because either the caller skipped
-  // comments and normally we won't be at the beginning of a comment token
-  // (making this option irrelevant), or the caller lexed comments and
-  // we need to lex just the comment token.
-  Lexer L(FakeLangOpts, SM, BufferID, nullptr, LexerMode::Swift,
-          HashbangMode::Allowed, CommentRetentionMode::ReturnAsTokens);
-  L.restoreState(State(Loc));
-  L.skipToEndOfLine(/*EatNewline=*/true);
-  return getSourceLoc(L.CurPtr);
+  // Windows line endings are \r\n. Since we want the start of the next
+  // line, just look for \n so the \r is skipped through.
+  size_t Offset = SM.getLocOffsetInBuffer(Loc, BufferID);
+  Offset = Buffer.find('\n', Offset);
+  if (Offset == StringRef::npos)
+    return SourceLoc();
+  return getSourceLoc(Buffer.data() + Offset + 1);
 }
 
 StringRef Lexer::getIndentationForLine(SourceManager &SM, SourceLoc Loc,