Xcode: Support XCODE_ATTRIBUTE_ with [variant=<config>] (#12532)

Since commit c519bb2b (XCode: Also qoute [] as needed to set
build-configurations, 2011-04-05) we escape "[]" conditions for
XCODE_ATTRIBUTE_ settings.  However, we need to handle the "variant"
condition with a special case to map it only into the settings for the
matching configuration.
Merge topic 'xcode-attributes-variant' into next

332350b Xcode: Support XCODE_ATTRIBUTE_ with [variant=<config>] (#12532)