[clang-tidy] Fix a crash for raw-string-literal check.

[hokein]

Summary

getSourceText could return an empty string for error cases (e.g. invalid
source locaiton), this patch makes the code more robust.

The crash did happen in our internal codebase, but unfortunately I
didn't manage to get a reproduce case. One thing I can confirm from
the core dump is that the crash is caused by calling isRawStringLiteral
on an empty Text.

clang-tools-extra/clang-tidy/modernize/RawStringLiteralCheck.cpp

for (const unsigned char C : Literal->getBytes())
     if (DisallowedChars.test(C))
       return false;
 
   CharSourceRange CharRange = Lexer::makeFileCharRange(
       CharSourceRange::getTokenRange(Literal->getSourceRange()),
       *Result.SourceManager, Result.Context->getLangOpts());
   StringRef Text = Lexer::getSourceText(CharRange, *Result.SourceManager,
                                         Result.Context->getLangOpts());
-  if (isRawStringLiteral(Text))
+  if (Text.empty() || isRawStringLiteral(Text))
     return false;
 
   return containsEscapes(Text, R"('\"?x01)");
 }
 
 bool containsDelimiter(StringRef Bytes, const std::string &Delimiter) {
   return Bytes.find(Delimiter.empty()
                         ? std::string(R"lit()")lit")