Bug 1854305 - Don't enable thread safety annotations before clang 11. r=nika

Arbitrary capability names weren't allowed until clang 11, and while we
could probably make things somehow work with earlier versions of clang,
it's not that interesting, as long as we keep the thread safety analysis
running with newer versions of clang on our CI (which we obviously do).

Differential Revision: https://phabricator.services.mozilla.com/D188791

mfbt/ThreadSafety.h

@@ -22,7 +22,7 @@
 #define mozilla_ThreadSafety_h
 #include "mozilla/Attributes.h"
 
-#if defined(__clang__) && (__clang_major__ >= 8) && !defined(SWIG)
+#if defined(__clang__) && (__clang_major__ >= 11) && !defined(SWIG)
 #  define MOZ_THREAD_ANNOTATION_ATTRIBUTE__(x) __attribute__((x))
 // Allow for localized suppression of thread-safety warnings; finer-grained
 // than MOZ_NO_THREAD_SAFETY_ANALYSIS