`_swift_willThrow` is not atomic.

[grynspan]

Description

The runtime exports a symbol, _swift_willThrow, for use by testing frameworks (specifically XCTest) to track when errors are thrown for diagnostic purposes. This symbol is not declared as an atomic value, which means that access to it has undefined behavior across threads. Because test logic can run concurrently (and that includes throwing errors), atomic access to this variable is necessary.

Steps to reproduce

N/A

Expected behavior

N/A