Avoid implicitly linking RegexBuilder symbol

[jmschonfeld]

Recently, FoundationEssentials began importing the RegexBuilder module. While it's an internal import, since we don't have the MemberImportVisibility feature flag enabled, this import affected overload resolution of this call to split. Eventually, we should enable that feature flag to avoid these types of issues, but for now we make the argument explicitly Character to pick the stdlib overload instead of the RegexBuilder overload to fix linking failures.

[jmschonfeld]

@swift-ci please test