[Backtracing] For DWARF 3, skip maximum_operations_per_instruction

DWARF 3 doesn't have the `maximum_operations_per_instruction` field
in the line number header.  As a result, we were mis-reading the
subsequent fields in the DWARF line table header.

rdar://168996222

Author: al45tair

stdlib/public/RuntimeModule/Dwarf.swift

@@ -851,8 +851,13 @@ struct DwarfReader<S: DwarfSource & AnyObject> {
       // .6 minimum_instruction_length
       let minimumInstructionLength = UInt(try cursor.read(as: Dwarf_Byte.self))
 
-      // .7 maximum_operations_per_instruction
-      let maximumOpsPerInstruction = UInt(try cursor.read(as: Dwarf_Byte.self))
+      let maximumOpsPerInstruction: UInt
+      if version >= 4 {
+        // .7 maximum_operations_per_instruction
+        maximumOpsPerInstruction = UInt(try cursor.read(as: Dwarf_Byte.self))
+      } else {
+        maximumOpsPerInstruction = 1
+      }
 
       // .8 default_is_stmt
       let defaultIsStmt = try cursor.read(as: Dwarf_Byte.self) != 0