DWARF: Never emit (0, 0) to mean an empty span in debug_loc (#2940)

After mapping to the new positions, and after relativizing to the
base, if we end up with (0, 0) then we must emit something else, as
that would be interpreted as the end of a list. As it is an empty
span, the actual value doesn't matter, it just has to be != 0.
This can happen if the very first span in a compile unit is an
empty span, in which case relative to the base of the compile unit
we would have (0, 0).

Author: kripken

src/wasm/wasm-debug.cpp

@@ -1001,6 +1001,16 @@ static void updateLoc(llvm::DWARFYAML::Data& yaml,
         assert(newStart >= newBase && newEnd >= newBase);
         newStart -= newBase;
         newEnd -= newBase;
+        if (newStart == 0 && newEnd == 0) {
+          // After mapping to the new positions, and after relativizing to the
+          // base, if we end up with (0, 0) then we must emit something else, as
+          // that would be interpreted as the end of a list. As it is an empty
+          // span, the actual value doesn't matter, it just has to be != 0.
+          // This can happen if the very first span in a compile unit is an
+          // empty span, in which case relative to the base of the compile unit
+          // we would have (0, 0).
+          newStart = newEnd = IGNOREABLE_LOCATION;
+        }
       }
       // The loc start and end markers have been preserved. However, TODO
       // instructions in the middle may have moved around, making the loc no