Add assert on End iteration distance to Rewriter::getRewrittenText.

[ahedberg]

Summary

I currently have code that is crashing in the second std::advance call,
and it was not straightforward to identify the problem, as the first line
of the stacktrace is in RopePieceBTreeIterator::operator++:

*** SIGILL; stack trace: ***
PC: clang/include/clang/Rewrite/Core/RewriteRope.h:119 clang::RopePieceBTreeIterator::operator++()
    ../include/c++/v1/__iterator/advance.h:35 std::__u::__advance<>()
    ../include/c++/v1/__iterator/advance.h:65 std::__u::advance<>()
    clang/lib/Rewrite/Rewriter.cpp:228 clang::Rewriter::getRewrittenText()
    clang/include/clang/Rewrite/Core/Rewriter.h:106 clang::Rewriter::getRewrittenText()

Adding an assertion produces a friendlier error message for the caller.

clang/lib/Rewrite/Rewriter.cpp

std::string Rewriter::getRewrittenText(CharSourceRange Range) const {
   // start of the last token.
   if (Range.isTokenRange())
     EndOff += Lexer::MeasureTokenLength(Range.getEnd(), *SourceMgr, *LangOpts);
 
   // Advance the iterators to the right spot, yay for linear time algorithms.
   RewriteBuffer::iterator Start = RB.begin();
   std::advance(Start, StartOff);
   RewriteBuffer::iterator End = Start;
+  assert(EndOff >= StartOff && "Invalid iteration distance");
   std::advance(End, EndOff-StartOff);
 
   return std::string(Start, End);
 }
 
 unsigned Rewriter::getLocationOffsetAndFileID(SourceLocation Loc,
                                               FileID &FID) const {
   assert(Loc.isValid() && "Invalid location");