Make ifThrown work in @safe code

[dlangBugzillaToGithub]

Bastiaan (@veelo) reported this on 2019-03-15T20:13:24Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=19741

Description

std.exception.ifThrown doesn’t work in @safe code: https://run.dlang.io/is/d0oTNi

H.S.Teoh has done some digging for the why of it: https://forum.dlang.org/post/mailman.7542.1552677586.29801.digitalmars-d-learn@puremagic.com