[clang] Add test for CWG203 "Type of address-of-member expression" (#121687)
This patch adds test for
[CWG203](https://cplusplus.github.io/CWG/issues/203.html). Author was
asking to change the type of pointer-to-member expression to be closer
to how it's written as opposed to where the resulting member belongs to,
but was turned down due to backwards compatibility concerns, so we're
testing the status quo.
There are a total of 6 examples in the filing, so I decided to just
throw all of them into the test. I had to turn example 2 into
`constexpr` test that unfortunately requires C++20. Outcomes in example
5 that Tomasz expected are not in line with implementation behavior and
my reading of the Standard. I think he got confused by the fact that
unlike regular pointers, pointers-to-members can be implicitly
_downcasted_, but not upcasted. I left comments in the example.
llvm-project
10fb5d2b - [clang] Add test for CWG203 "Type of address-of-member expression" (#121687)
