Fix #20372: Check pattern match exhaustivity in inlined code

[jchyb]

Fixes #20372
c0e93f1 previously disabled warnings for pattern matches in Inlined code, as things like:

inline def count(inline x: Boolean) = x match
    case true => 1
    case false => 0
count(true) // inlined to true match {case true => 1; case false => 0 }

would throw warnings about unreachable cases, which could have been confusing to the users. However, in those cases it should be enough to disallow checks for unreachable cases specifically, and leave exhaustivity checks intact.