swift234

[Coverage] Fix handling of return statements within repeat-while loops

In 2a34b7c6, we introduced an incorrect test case for return statements
within repeat-while loops. Consider this situation:

repeat {     // Region 1
  return
} while C1   // Should be "zero", not "Region 1"

The fix requires maintaining a stack of active repeat-while loops. This
is an accepted idiom (c.f the clang CoverageMapping implementation). To
see why a stack is needed, consider:

repeat {       // Region 1
  repeat {     // Region 2
    if (C1) {  // Region 3
      return
    }
  } while C2   // Should be "Region 2 - *Region 3*", not "Region 2"
} while C3     // Should be "Region 1 - *Region 3*", not "Region 1"

rdar://problem/24572268