Fixes regression introduced in b3e6ff331925dde24a4707452d657da0fdf7f588
Fixes bot failure https://lab.llvm.org/buildbot/#/builders/60/builds/15037
```
.---command stderr------------
| error: 'supported-warning' diagnostics seen but not expected:
| File C:\buildbot\as-builder-1\x-armv7l\llvm-project\clang\test\SemaCXX\ms-constexpr-new.cpp Line 7: '__cdecl' calling convention is not supported for this target
| 1 error generated.
`-----------------------------
```
This commit introduces support for the MSVC-specific C++11-style
attribute `[[msvc::constexpr]]`, which was introduced in MSVC 14.33.
The semantics of this attribute are enabled only under
MSVC compatibility (`-fms-compatibility-version`) 14.33 and higher.
Additionally, the default value of `_MSC_VER` has been raised to 1433.
The current implementation lacks support for:
- `[[msvc::constexpr]]` constructors (see #72149);
at the time of this implementation, such support would have required
an unreasonable number of changes in Clang.
- `[[msvc::constexpr]] return ::new` (constexpr placement new) from
non-std namespaces (see #74924).
Relevant to: #57696
