[analyzer] Remove overzealous "No dispatcher registered" assertion

Random testing revealed it's possible to crash the analyzer with the
command line invocation:

clang -cc1 -analyze -analyzer-checker=nullability empty.c

The assert in CheckerManager.cpp was deemed to be too strict so is removed.

clang: <root>/clang/lib/StaticAnalyzer/Core/CheckerManager.cpp:56:
   void clang::ento::CheckerManager::finishedCheckerRegistration():
     Assertion `Event.second.HasDispatcher && "No dispatcher registered for an event"' failed.

PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/

Stack dump:
0.      Program arguments: clang -cc1 -analyze -analyzer-checker=nullability nullability-nocrash.c
 #0 ...
 ...
 #7 <addr> clang::ento::CheckerManager::finishedCheckerRegistration()
 #8 <addr> clang::ento::CheckerManager::CheckerManager(clang::ASTContext&,
             clang::AnalyzerOptions&, clang::Preprocessor const&,
             llvm::ArrayRef<std::__cxx11::basic_string<char, std::char_traits<char>,
             std::allocator<char>>>, llvm::ArrayRef<std::function<void (clang::ento::CheckerRegistry&)>>)

This commit removes the assertion which failed here, because it was
logically incorrect: it required that if an Event is handled by some
(enabled) checker, then there must be an enabled checker which can
emit that kind of Event. It should be OK to disable the event-producing
checkers but enable an event-consuming checker which has different
responsibilities in addition to handling the events.

Note that this assertion was in an #ifndef NDEBUG block, so this
change does not impact the non-debug builds.

Author: Vince Bridgers

clang/include/clang/StaticAnalyzer/Core/CheckerManager.h

@@ -164,8 +164,6 @@ class CheckerManager {
 
   bool hasPathSensitiveCheckers() const;
 
-  void finishedCheckerRegistration();
-
   const LangOptions &getLangOpts() const { return LangOpts; }
   const AnalyzerOptions &getAnalyzerOptions() const { return AOptions; }
   const Preprocessor &getPreprocessor() const {

clang/lib/StaticAnalyzer/Core/CheckerManager.cpp

@@ -48,16 +48,6 @@ bool CheckerManager::hasPathSensitiveCheckers() const {
       EvalCallCheckers, EndOfTranslationUnitCheckers);
 }
 
-void CheckerManager::finishedCheckerRegistration() {
-#ifndef NDEBUG
-  // Make sure that for every event that has listeners, there is at least
-  // one dispatcher registered for it.
-  for (const auto &Event : Events)
-    assert(Event.second.HasDispatcher &&
-           "No dispatcher registered for an event");
-#endif
-}
-
 void CheckerManager::reportInvalidCheckerOptionValue(
     const CheckerBase *C, StringRef OptionName,
     StringRef ExpectedValueDesc) const {

clang/lib/StaticAnalyzer/Frontend/CreateCheckerManager.cpp

@@ -28,7 +28,6 @@ CheckerManager::CheckerManager(
                            AOptions, checkerRegistrationFns);
   Registry.initializeRegistry(*this);
   Registry.initializeManager(*this);
-  finishedCheckerRegistration();
 }
 
 CheckerManager::CheckerManager(AnalyzerOptions &AOptions,

clang/test/Analysis/nullability-nocrash.c

@@ -0,0 +1,13 @@
+// RUN: %clang_analyze_cc1 -w -analyzer-checker=nullability \
+// RUN:                       -analyzer-output=text -verify %s
+//
+// expected-no-diagnostics
+//
+// Previously there was an assertion requiring that if an Event is handled by
+// some enabled checker, then there must be at least one enabled checker which
+// can emit that kind of Event.
+// This assertion failed when NullabilityChecker (which is a subclass of
+// check::Event<ImplicitNullDerefEvent>) was enabled, but the checkers
+// inheriting from EventDispatcher<ImplicitNullDerefEvent> were all disabled.
+// This test file validates that enabling the nullability checkers (without any
+// other checkers) no longer causes a crash.