Unlike C++, C allows the definition of an uninitialized `const` object.
If the object has static or thread storage duration, it is still
zero-initialized, otherwise, the object is left uninitialized. In either
case, the code is not compatible with C++.
This adds a new diagnostic group, `-Wdefault-const-init-unsafe`, which
is on by default and diagnoses any definition of a `const` object which
remains uninitialized.
It also adds another new diagnostic group, `-Wdefault-const-init` (which
also enabled the `unsafe` variant) that diagnoses any definition of a
`const` object (including ones which are zero-initialized). This
diagnostic is off by default.
Finally, it adds `-Wdefault-const-init` to `-Wc++-compat`. GCC diagnoses
these situations under this flag.
Fixes#19297
576161cb60
