[clang] Add tests for CWG issues regarding completeness of types (#92113)

This patch covers the following Core issues:
[CWG930](https://cplusplus.github.io/CWG/issues/930.html) "`alignof`
with incomplete array type"
[CWG1110](https://cplusplus.github.io/CWG/issues/1110.html) "Incomplete
return type should be allowed in `decltype` operand"
[CWG1340](https://cplusplus.github.io/CWG/issues/1340.html) "Complete
type in member pointer expressions"
[CWG1352](https://cplusplus.github.io/CWG/issues/1352.html)
"Inconsistent class scope and completeness rules"
[CWG1458](https://cplusplus.github.io/CWG/issues/1458.html) "Address of
incomplete type vs `operator&()`"
[CWG1824](https://cplusplus.github.io/CWG/issues/1824.html)
"Completeness of return type vs point of instantiation"
[CWG1832](https://cplusplus.github.io/CWG/issues/1832.html) "Casting to
incomplete enumeration"
[CWG2304](https://cplusplus.github.io/CWG/issues/2304.html) "Incomplete
type vs overload resolution"
[CWG2310](https://cplusplus.github.io/CWG/issues/2310.html) "Type
completeness and derived-to-base pointer conversions"
[CWG2430](https://cplusplus.github.io/CWG/issues/2430.html)
"Completeness of return and parameter types of member functions"
[CWG2512](https://cplusplus.github.io/CWG/issues/2512.html) "`typeid`
and incomplete class types"
[CWG2630](https://cplusplus.github.io/CWG/issues/2630.html) "Syntactic
specification of class completeness"
[CWG2718](https://cplusplus.github.io/CWG/issues/2718.html) "Type
completeness for derived-to-base conversions"
[CWG2857](https://cplusplus.github.io/CWG/issues/2857.html)
"Argument-dependent lookup with incomplete class types"

Current wording for CWG1110 came from
[P0135R1](https://wg21.link/p0135R1) "Wording for guaranteed copy
elision through simplified value categories".

As a drive-by fix, I fixed incorrect status of CWG1815, test for which
was added in #87933. CC @yronglin