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.
Reviewed By: gribozavr2
Differential Revision: https://reviews.llvm.org/D117579
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);