This introduces a new diagnostic group to diagnose implicit casts from
int to an enumeration type. In C, this is valid, but it is not
compatible with C++.
Additionally, this moves the "implicit conversion from enum type to
different enum type" diagnostic from `-Wenum-conversion` to a new group
`-Wimplicit-enum-enum-cast`, which is a more accurate home for it.
`-Wimplicit-enum-enum-cast` is also under `-Wimplicit-int-enum-cast`, as
it is the same incompatibility (the enumeration on the right-hand is
promoted to `int`, so it's an int -> enum conversion).
Fixes#37027
df267d77f6
