Use a stable sort to handle overlapping/duplicate line sequences

This can occur due to linker ICF and stable sort will ensure the results
are stable.

No explicit/new test coverage, because nondeterminism is non-testable.

It should already be covered by the DWARFDebugLineTest that was failing
some internal testing on an ARM machine which might've been what changed
the sort order. But `llvm::sort` also deliberately randomizes the
contents (under EXPENSIVE_CHECKS) so I'd have expected failures to show
up in any EXPENSIVE_CHECKS Build...

Author: dwblaikie

llvm/lib/DebugInfo/DWARF/DWARFDebugLine.cpp

@@ -1274,13 +1274,14 @@ Error DWARFDebugLine::LineTable::parse(
 
   // Sort all sequences so that address lookup will work faster.
   if (!Sequences.empty()) {
-    llvm::sort(Sequences, Sequence::orderByHighPC);
+    llvm::stable_sort(Sequences, Sequence::orderByHighPC);
     // Note: actually, instruction address ranges of sequences should not
     // overlap (in shared objects and executables). If they do, the address
     // lookup would still work, though, but result would be ambiguous.
     // We don't report warning in this case. For example,
     // sometimes .so compiled from multiple object files contains a few
     // rudimentary sequences for address ranges [0x0, 0xsomething).
+    // Address ranges may also overlap when using ICF.
   }
 
   // Terminate the table with a final blank line to clearly delineate it from