[clang] Avoid accessing unset optional, workaround for #100095 (PR #100408)

[Alexander Kornienko via cfe-commits]

alexfh wrote:

> Could we merge the test into this change? I think we could then land this as a temporary workaround indeed. I'll keep looking into fixing this proper and will remove the XFAIL line when appropriate.

I've added the test to the change and verified it:
* fails  without the SemaTemplateDeduction.cpp change, when compiled against libc++ with hardening enabled and with LLVM assertions disabled
* passes with the SemaTemplateDeduction.cpp change, when compiled against libc++ with hardening enabled and with LLVM assertions disabled
* XFAILs with the SemaTemplateDeduction.cpp change, when compiled against libc++ with hardening enabled and with LLVM assertions enabled.

Unfortunately, we don't have a buildbot with LLVM tests built against libc++ + hardening, but we're going to test it routinely internally at Google.

https://github.com/llvm/llvm-project/pull/100408