[llvm][FileCheck] Fix unit tests failures with EXPENSIVE_CHECKS

[DavidSpickett]

Summary

EXPENSIVE_CHECKS enables _GLIBCXX_DEBUG, which makes std::sort
check that the compare function is implemented correctly.

To do this it calls it with the first item as both sides.
Which trips the assert here because we think they're
2 capture ranges that overlap, when it's just the same range twice.

Check up front for the two sides being the same item
(same address, not just ==).

[DavidSpickett]

To give some context, this small example demos what I mean:

#define _GLIBCXX_DEBUG
#include <algorithm>
#include <array>
#include <stdio.h>

int main()
{
    std::array<int, 1> s = {5};
    std::sort(s.begin(), s.end(), [](int a, int b) {
        printf("comparator called!\n");
        return a < b;
    });
}

On a couple of systems I have access to, that will print even though there's only 1 item.

There is a buildbot for EXPENSIVE_CHECKS but it is release mode and on Debian. So it's possible that changes if you hit this or not.

Anyway, hopefully the logic makes sense.

[kazu]

I am inclined to having the check unconditionally to be more robust. It is super cheap. The condition always evaluates to false without EXPENSIVE_CHECKS, so it gets along with the branch predictor. Plus, it would be nice not to rely on the fact that EXPENSIVE_CHECKS enables _GLIBCXX_DEBUG. Thoughts?

[DavidSpickett]

Always do the check.

[DavidSpickett]

I am inclined to having the check unconditionally to be more robust.

Sounds good to me, done.

[kazu]

LGTM. Thank you for updating the patch!

llvm/lib/FileCheck/FileCheck.cpp

for (const auto &VariableDef : NumericVariableDefs) {
     SMLoc Start = SMLoc::getFromPointer(StrValue->data());
     SMLoc End = SMLoc::getFromPointer(StrValue->data() + StrValue->size());
     VC.Range = SMRange(Start, End);
     VarCaptures.push_back(VC);
   }
   // Sort variable captures by the order in which they matched the input.
   // Ranges shouldn't be overlapping, so we can just compare the start.
   llvm::sort(VarCaptures, [](const VarCapture &A, const VarCapture &B) {
+    if (&A == &B)
+      return false;
     assert(A.Range.Start != B.Range.Start &&
            "unexpected overlapping variable captures");
     return A.Range.Start.getPointer() < B.Range.Start.getPointer();
   });
   // Create notes for the sorted captures.
   for (const VarCapture &VC : VarCaptures) {
     SmallString<256> Msg;
     raw_svector_ostream OS(Msg);