[clang] clang: Fix parsing of seh exception model (PR #146643)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-aarch64-darwin` running on `doug-worker-4` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/22666

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: Modules/mingw-exceptions.cppm' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
/Users/buildbot/buildbot-root/aarch64-darwin/build/bin/clang -target x86_64-windows-gnu -x c++-module -std=gnu++23 -c -o /dev/null -Xclang -disable-llvm-passes /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/clang/test/Modules/mingw-exceptions.cppm # RUN: at line 1
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/clang -target x86_64-windows-gnu -x c++-module -std=gnu++23 -c -o /dev/null -Xclang -disable-llvm-passes /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/clang/test/Modules/mingw-exceptions.cppm
error: unable to create target: 'No available targets are compatible with triple "x86_64-unknown-windows-gnu"'
1 error generated.

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/146643