[Concepts] Fix ConceptSpecializationExpr profiling crash

ConceptSpecializationExprs (CSEs) were being created with nullptr
TemplateArgsAsWritten during TemplateTemplateParmDecl canonicalization, and
we were relying on them during profiling which caused sporadic crashes
in test/CXX/.../temp.arg.template/p3-2a.cpp introduced in D44352.

Change profiling of CSEs to instead rely on the actual converted template
arguments and concept named.
diff --git a/clang/lib/AST/StmtProfile.cpp b/clang/lib/AST/StmtProfile.cpp
index 2aa5106..c0b0f3b 100644
--- a/clang/lib/AST/StmtProfile.cpp
+++ b/clang/lib/AST/StmtProfile.cpp
@@ -1335,9 +1335,9 @@
 void StmtProfiler::VisitConceptSpecializationExpr(
                                            const ConceptSpecializationExpr *S) {
   VisitExpr(S);
-  VisitDecl(S->getFoundDecl());
-  VisitTemplateArguments(S->getTemplateArgsAsWritten()->getTemplateArgs(),
-                         S->getTemplateArgsAsWritten()->NumTemplateArgs);
+  VisitDecl(S->getNamedConcept());
+  for (const TemplateArgument &Arg : S->getTemplateArguments())
+    VisitTemplateArgument(Arg);
 }
 
 static Stmt::StmtClass DecodeOperatorCall(const CXXOperatorCallExpr *S,