[6.1] Fix race condition in __CFStringGetEightBitStringEncoding

[jmschonfeld]

• Explanation: Fixes a small behavior issue caused by a race condition in __CFStringGetEightBitStringEncoding
• Scope: Impacts many of the low-level CFString functions that use CoreFoundation's "eight bit" string encoding
• Issues: String(format:) intermittently returns an empty string when run concurrently by swift-testing #5152
• Original PRs: Fix race condition in __CFStringGetEightBitStringEncoding #5155
• Risk: Medium - this is a low level function that is called from a lot of places in CoreFoundation, but the change is very small and is not supposed to change behavior here aside from the window of the race condition.
• Testing: Local testing at desk
• Reviewers: @parkera

[jmschonfeld]

@swift-ci please test