[Linux][Backtracing] Fix incorrect line number generation.

The code that looks up line numbers wasn't respecting the end-sequence
flag, so it could "find" a line number between the end of one sequence
and the beginning of another.  That was wrong.

rdar://112595022

Author: al45tair

stdlib/public/Backtracing/Elf.swift

@@ -1680,7 +1680,11 @@ class ElfImage<SomeImageSource: ImageSource,
           done = true
         }
 
-        prevState = state
+        if state.endSequence {
+          prevState = nil
+        } else {
+          prevState = state
+        }
       }
     }