Defer infer bool param

[som-snytt]

The previous tweak to take if (_) as if (_: Boolean) is too strict.

That disallowed

BooleanSetting().("-nowarn").withPostSetHook(if(_) maxwarns.value=0)

Follow-up to #7707

Noticed previously

This might be perfectly normal, but I wanted to do this, then remembered the param is a setting not a boolean, but I noticed the message is suboptimal:

[error] .../nsc/settings/StandardScalaSettings.scala:54:127: type mismatch;
[error]  found   : Boolean => Unit
[error]  required: stabilizer$1.type => Unit
[error]   val nowarn =         BooleanSetting ("-nowarn", "Generate no warnings.") withAbbreviation "--no-warnings" withPostSetHook { if (_) maxwarns.value = 0 }
[error]