Merge #14129: Trivial: update clang thread-safety docs url

Summary:
c7f7fa467ec571d3b3fd7ae1d32429b74b18ad9c Trivial: update clang thread-safety docs url (Ben Woosley)

Pull request description:

From the defunct http://clang.llvm.org/docs/LanguageExtensions.html#threadsafety to https://clang.llvm.org/docs/ThreadSafetyAnalysis.html

Tree-SHA512: 5113c3933fccee7b45cace5d8dffa38b46ed9ad1422795d57843a20b276ed0e513bbf8d3d2bd28f55a46baf14a349871d7a635485785b93cb21baf8b8720c15a

Backport of Core PR14129

Test Plan: Verify old link does not lead to information on thread safety whereas the new link does lead to a page about clang thread safety

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6226