[clang] [Clang] Added explanation why `is_constructible` evaluated to false. (PR #143309)

[Mikael Holmén via cfe-commits]

mikaelholmen wrote:

Hi @egorshamshura and @cor3ntin

It seems like a bunch of libc++ testcases start failing with this patch:
```
Failed Tests (9):
  llvm-libc++-shared.cfg.in :: libcxx/selftest/dsl/dsl.sh.py
  llvm-libc++-shared.cfg.in :: libcxx/utilities/expected/expected.expected/and_then.mandates.verify.cpp
  llvm-libc++-shared.cfg.in :: libcxx/utilities/expected/expected.expected/or_else.mandates.verify.cpp
  llvm-libc++-shared.cfg.in :: libcxx/utilities/expected/expected.expected/value.observers.verify.cpp
  llvm-libc++-shared.cfg.in :: libcxx/utilities/expected/expected.void/and_then.mandates.verify.cpp
  llvm-libc++-shared.cfg.in :: std/containers/sequences/array/array.creation/to_array.verify.cpp
  llvm-libc++-shared.cfg.in :: std/containers/views/mdspan/mdspan/conversion.verify.cpp
  llvm-libc++-shared.cfg.in :: std/utilities/function.objects/func.bind.partial/bind_back.verify.cpp
  llvm-libc++-shared.cfg.in :: std/utilities/function.objects/func.bind_front/bind_front.verify.cpp
```
The
```
  llvm-libc++-shared.cfg.in :: libcxx/selftest/dsl/dsl.sh.py
```
failure happens also before this patch, but the other ones are new.

One example of a failure printout for libcxx/utilities/expected/expected.expected/value.observers.verify.cpp:
```
# .---command stderr------------
# | error: 'expected-error' diagnostics seen but not expected: 
# |   File /repo/llvm/build-all-builtins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/libcxx/test-suite-install/include/c++/v1/__expected/expected.h Line 79: call to deleted constructor of 'MoveOnly'
# |   File /repo/llvm/build-all-builtins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/libcxx/test-suite-install/include/c++/v1/__expected/expected.h Line 79: call to deleted constructor of 'CopyConstructibleButNotMoveConstructible'
# | 2 errors generated.
# `-----------------------------
# error: command failed with exit status: 1
```

Seen e.g. here: https://lab.llvm.org/buildbot/#/builders/55/builds/12732

https://github.com/llvm/llvm-project/pull/143309