[clang] Implement `__is_layout_compatible` (#81506)

This patch implements `__is_layout_compatible` intrinsic, which supports
`std::is_layout_compatible` type trait introduced in C++20
([P0466R5](https://wg21.link/p0466r5) "Layout-compatibility and
Pointer-interconvertibility Traits"). Name matches GCC and MSVC
intrinsic.

Basically, this patch exposes our existing machinery for checking for
layout compatibility and figuring out common initial sequences. Said
machinery is a bit outdated, as it doesn't implement
[CWG1719](https://cplusplus.github.io/CWG/issues/1719.html) "Layout
compatibility and cv-qualification revisited" and
[CWG2759](https://cplusplus.github.io/CWG/issues/2759.html)
"`[[no_unique_address]` and common initial sequence". Those defect
reports are considered out of scope of of this PR, but will be
implemented in subsequent PRs.

Partially addresses #48204