[APINotes] Avoid "two entries for the same version" failure with expensive checks

Found assertion failures when using EXPENSIVE_CHECKS and running
lit tests for APINotes. It seems like std::is_sorted is verifying
that the comparison function is reflective (comp(a,a)=false) when
using expensive checks. So we would get callbacks to the lambda
used for comparison, even for vectors with a single element
in APINotesReader::VersionedInfo<T>::VersionedInfo, with "left"
and "right" being the same object. Therefore the assert checking
that we never found equal values would fail.

Fix makes sure that we skip the check for equal values when
"left" and "right" is the same object.

Author: bjope

clang/lib/APINotes/APINotesReader.cpp

@@ -2045,7 +2045,12 @@ APINotesReader::VersionedInfo<T>::VersionedInfo(
       Results.begin(), Results.end(),
       [](const std::pair<llvm::VersionTuple, T> &left,
          const std::pair<llvm::VersionTuple, T> &right) -> bool {
-        assert(left.first != right.first && "two entries for the same version");
+        // The comparison function should be reflective, and with expensive
+        // checks we can get callbacks basically checking that lambda(a,a) is
+        // false. We could still check that we do not find equal elements when
+        // left!=right.
+        assert((&left == &right || left.first != right.first) &&
+               "two entries for the same version");
         return left.first < right.first;
       }));