[C23] Implement N3018: The constexpr specifier for object definitions (#73099)

The implementation mostly reuses C++ code paths where possible,
including narrowing check in order to provide diagnostic messages in
case initializer for constexpr variable is not exactly representable in
target type.

The following won't work due to lack of support for other features:
- Diagnosing of underspecified declarations involving constexpr
- Constexpr attached to compound literals

Also due to lack of support for char8_t some of examples with utf-8
strings don't work properly.

Fixes https://github.com/llvm/llvm-project/issues/64742