[clang] SwiftCallingConv: Fix the splitVectorEntry function (PR #69953)

[Arnold Schwaighofer via cfe-commits]

aschwaighofer wrote:

The failures are not related to this PR / pre-existing.

`clang/docs/ReleaseNotes.rst:` is not touched by this PR.

```
+ echo '*** Checking for trailing whitespace left in Clang source files ***'
--
  | *** Checking for trailing whitespace left in Clang source files ***
  | + grep -rnI '[[:blank:]] clang/lib clang/include clang/docs
  | clang/docs/ReleaseNotes.rst:108:  when targetting MSVC to match the behavior of MSVC.
  | + echo '*** Trailing whitespace has been found in Clang source files as described above ***'
  | *** Trailing whitespace has been found in Clang source files as described above ***
```

And the `CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp` failure also occurs in the baseline.



https://github.com/llvm/llvm-project/pull/69953