[clang] [Clang] prevent null explicit object argument from being deduced (PR #104328)

[via cfe-commits]

https://github.com/cor3ntin commented:

Fix LGTM
Can you move the test to test/SemaCXX/cxx2b-deducing-this.cpp, 
in a `GH102025` namespace?

https://github.com/llvm/llvm-project/pull/104328