[analyzer] Print the offending function at EndAnalysis crash

[steakhal]

Summary

I've faced crashes in the past multiple times when some
check::EndAnalysis callback caused some crash.
It's really anoying that it doesn't tell which function triggered this
callback.

This patch adds the well-known trace for that situation as well.
Example:

1.      <eof> parser at end of file
2.      While analyzing stack:
        #0 Calling test11

Note that this does not have tests.
I've considered unittests for this purpose, by using the
ASSERT_DEATH() similarly how we check double eval called functions in
ConflictingEvalCallsTest.cpp, however, that the testsuite won't invoke
the custom handlers. Only the message of the llvm_unreachable() will
be printed. Consequently, it's not applicable for us testing this
feature.

I've also considered using an end-to-end LIT test for this.
For that, we would need to somehow overload the clang_analyzer_crash()
ExprInspection handler, to get triggered by other events than the
EvalCall. I'm not saying that we could not come up with a generic way
of causing crash in a specific checker callback, but I'm not sure if
that would worth the effort.

[martong]

clang/lib/StaticAnalyzer/Core/ExprEngine.cpp

616

Okay, so, at least we will get the name of the top-level function, right? Some comments might be useful here.

[steakhal]

• Add a comment to the trace builder.

[martong]

LGTM!

clang/include/clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h

public:
 
   const NodeBuilderContext &getBuilderContext() {
     assert(currBldrCtx);
     return *currBldrCtx;
   }
 
   const Stmt *getStmt() const;
 
+  const LocationContext *getRootLocationContext() const {
+    assert(G.roots_begin() != G.roots_end());
+    return (*G.roots_begin())->getLocation().getLocationContext();
+  }
+
   void GenerateAutoTransition(ExplodedNode *N);
   void enqueueEndOfPath(ExplodedNodeSet &S);
   void GenerateCallExitNode(ExplodedNode *N);
 
 
   /// Dump graph to the specified filename.
   /// If filename is empty, generate a temporary one.
   /// \return The filename the graph is written into.

clang/lib/StaticAnalyzer/Core/ExprEngine.cpp

if (LCtx && !State->get<ObjectsUnderConstruction>().isEmpty()) {
     Out << "null," << NL;
   }
 
   getCheckerManager().runCheckersForPrintStateJson(Out, State, NL, Space,
                                                    IsDot);
 }
 
 void ExprEngine::processEndWorklist() {
+  // This prints the name of the top-level function if we crash.
+  PrettyStackTraceLocationContext CrashInfo(getRootLocationContext());
   getCheckerManager().runCheckersForEndAnalysis(G, BR, *this);
 }
 
 void ExprEngine::processCFGElement(const CFGElement E, ExplodedNode *Pred,
                                    unsigned StmtIdx, NodeBuilderContext *Ctx) {
   PrettyStackTraceLocationContext CrashInfo(Pred->getLocationContext());
   currStmtIdx = StmtIdx;
   currBldrCtx = Ctx;