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

c7f7fa467e 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

src/threadsafety.h

@@ -10,7 +10,7 @@
 // TL;DR Add GUARDED_BY(mutex) to member variables. The others are
 // rarely necessary. Ex: int nFoo GUARDED_BY(cs_foo);
 //
-// See http://clang.llvm.org/docs/LanguageExtensions.html#threadsafety
+// See https://clang.llvm.org/docs/ThreadSafetyAnalysis.html
 // for documentation.  The clang compiler can do advanced static analysis
 // of locking when given the -Wthread-safety option.
 #define LOCKABLE __attribute__((lockable))