Fix buffer overflow in Lexer

[kcc]

Summary

Fix PR22407, where the Lexer overflows the buffer when parsing
#include<\
(end of file after slash)

[rnk]

lgtm

lib/Lex/Lexer.cpp

 /// after having lexed the '<' character.  This is used for #include filenames.
 bool Lexer::LexAngledStringLiteral(Token &Result, const char *CurPtr) {
   // Does this string contain the \0 character?
   const char *NulCharacter = nullptr;
   const char *AfterLessPos = CurPtr;
   char C = getAndAdvanceChar(CurPtr, Result);
   while (C != '>') {
     // Skip escaped characters.
-    if (C == '\\') {
+    if (C == '\\' && CurPtr < BufferEnd) {
       // Skip the escaped character.
       getAndAdvanceChar(CurPtr, Result);
     } else if (C == '\n' || C == '\r' ||             // Newline.
                (C == 0 && (CurPtr-1 == BufferEnd ||  // End of file.
                            isCodeCompletionPoint(CurPtr-1)))) {
       // If the filename is unterminated, then it must just be a lone <
       // character.  Return this as such.
       FormTokenWithChars(Result, AfterLessPos, tok::less);

test/Lexer/eof-include.c

+// RUN: %clang_cc1 %s -verify
+// vim: set binary noeol:
+
+// This file intentionally ends without a \n on the last line.  Make sure your
+// editor doesn't add one.
+
+// expected-error@+1{{expected "FILENAME" or <FILENAME>}}
+#include <\
 No newline at end of file