[clang] [llvm] [WinEH] Fix crash, object unwinding in the except block (PR #172287)

[via llvm-commits]

MuellerMP wrote:

My general take on this issue: since msvc rejects this it would not be unreasonable to reject this in clang aswell for compatibility reasons and to avoid overall complexity in the state handling.

Regarding this patch: 
The C++ state numbering condition aswell as the Filter state numbering also is not required for this issue and are not covered by tests. 
As far as I can tell the only fix covered by the test is the SEH state numbering `State > 0` condition for the seh_try_end invoke right?

Now what actually causes the issue:
We choose the `calculateSEHStateNumbers` function instead of the `calculateWinCXXEHStateNumbers` Function in FunctionLoweringInfo due to this function having the `__C_specific_handler` personality.
This function currently does **NOT** know about C++ EH constructs like the emitted `seh_scope_begin`. We simply skip over it in the state numbering which causes the issue.
We could "fix" this by either supporting `seh_scope_begin` in `calculateSEHStateNumbers` or choosing the C++ unwinder in the frontend (latter one would be preferable and more sound IMO).

=>But like I said: I would rather choose to reject this in the frontend.

https://github.com/llvm/llvm-project/pull/172287