Thread Safety Analysis: Graduate ACQUIRED_BEFORE() and ACQUIRED_AFTER() from beta features (llvm#152853)

Both these attributes were introduced in ab1dc2d ("Thread Safety
Analysis: add support for before/after annotations on mutexes") back in
2015 as "beta" features.

Anecdotally, we've been using `-Wthread-safety-beta` for years without
problems.

Furthermore, this feature requires the user to explicitly use these
attributes in the first place.

After 10 years, let's graduate the feature to the stable feature set,
and reserve `-Wthread-safety-beta` for new upcoming features.

Author: melver

clang/docs/ThreadSafetyAnalysis.rst

@@ -825,13 +825,6 @@ doesn't know that munl.mu == mutex.  The SCOPED_CAPABILITY attribute handles
 aliasing for MutexLocker, but does so only for that particular pattern.
 
 
-ACQUIRED_BEFORE(...) and ACQUIRED_AFTER(...) support is still experimental.
----------------------------------------------------------------------------
-
-ACQUIRED_BEFORE(...) and ACQUIRED_AFTER(...) are currently being developed under
-the ``-Wthread-safety-beta`` flag.
-
-
 .. _mutexheader:
 
 mutex.h

clang/lib/Analysis/ThreadSafety.cpp

@@ -1392,8 +1392,7 @@ void ThreadSafetyAnalyzer::addLock(FactSet &FSet, const FactEntry *Entry,
   }
 
   // Check before/after constraints
-  if (Handler.issueBetaWarnings() &&
-      !Entry->asserted() && !Entry->declared()) {
+  if (!Entry->asserted() && !Entry->declared()) {
     GlobalBeforeSet->checkBeforeAfter(Entry->valueDecl(), FSet, *this,
                                       Entry->loc(), Entry->getKind());
   }