MFH: r474545 r474546

www/firefox: switch to Clang 6.0.1 on FreeBSD 11.1

Clang 6 is already used on 10.4, 11.2 and -CURRENT. No need to expose
users to Clang 4.0.0 regressions fixed in 4.0.1 or later versions.

Approved by: ports-secteam blanket