[llvm] CoverageMappingWriter: Emit `Decision` before `Expansion` (PR #78966)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

The testcase (was in the initial description)

(https://godbolt.org/z/orE99Y7hT)
```
foo:
  File 0, 3:23 -> 5:2 = #0
  Expansion,File 0, 4:11 -> 4:12 = #0 (Expanded file = 1)
  Decision,File 0, 4:11 -> 4:20 = M:0, C:2
  Expansion,File 0, 4:19 -> 4:20 = #1 (Expanded file = 2)
  File 1, 1:14 -> 1:17 = #0
  File 1, 1:14 -> 1:17 = #0
  Branch,File 1, 1:14 -> 1:17 = #1, (#0 - #1) [1,2,0] 
  File 2, 1:14 -> 1:17 = #1
  Branch,File 2, 1:14 -> 1:17 = #2, (#1 - #2) [2,0,0] 
```

https://github.com/llvm/llvm-project/pull/78966