[llvm] Fix some test failures with EXPENSIVE_CHECKS and libstdc++

[DavidSpickett]

Summary

DebugLocEntry assumes that it either contains 1 item that has no fragment
or many items that all have fragments (see the assert in addValues).

When EXPENSIVE_CHECKS is enabled, _GLIBCXX_DEBUG is defined. On a few machines
I've checked, this causes std::sort to call the comparator even
if there is only 1 item to sort. Perhaps to check that it is implemented
properly ordering wise, I didn't find out exactly why.

operator< for a DbgValueLoc will crash if this happens because the
optional Fragment is empty.

Compiler/linker/optimisation level seems to make this happen
or not. So I've seen this happen on x86 Ubuntu but the buildbot
for release EXPENSIVE_CHECKS did not have this issue.

Add an explicit check whether we have 1 item.

[aprantl]

Shoud we just leave the check in unconditionally as an optimization?

llvm/lib/CodeGen/AsmPrinter/DebugLocEntry.h

220

presumably

[DavidSpickett]

Always check for 1 item.

llvm/lib/CodeGen/AsmPrinter/DebugLocEntry.h

void addValues(ArrayRef<DbgValueLoc> Vals) {
     assert((Values.size() == 1 || all_of(Values, [](DbgValueLoc V) {
               return V.isFragment();
             })) && "must either have a single value or multiple pieces");
   }
 
   // Sort the pieces by offset.
   // Remove any duplicate entries by dropping all but the first.
   void sortUniqueValues() {
+    // Values is either 1 item that does not have a fragment, or many items
+    // that all do. No need to sort if the former and also prevents operator<
+    // being called on a non fragment item when _GLIBCXX_DEBUG is defined.
+    if (Values.size() == 1)
+      return;
     llvm::sort(Values);
     Values.erase(std::unique(Values.begin(), Values.end(),
                              [](const DbgValueLoc &A, const DbgValueLoc &B) {
                                return A.getExpression() == B.getExpression();
                              }),
                  Values.end());
   }