[Clang] Fix a crash on invalid concept (llvm#150186)

Fixes llvm#149986

Author: cor3ntin

clang/lib/Sema/SemaConcept.cpp

@@ -588,6 +588,9 @@ static bool CheckConstraintSatisfaction(
     return true;
 
   for (const AssociatedConstraint &AC : AssociatedConstraints) {
+    if (AC.isNull())
+      return true;
+
     Sema::ArgPackSubstIndexRAII _(S, AC.ArgPackSubstIndex);
     ExprResult Res = calculateConstraintSatisfaction(
         S, Template, TemplateIDRange.getBegin(), TemplateArgsLists,

clang/test/SemaTemplate/concepts.cpp

@@ -1250,3 +1250,11 @@ static_assert(!D<Priv>::has, "Private should be invisible.");
 static_assert(!D<Prot>::has, "Protected should be invisible.");
 
 }
+
+
+namespace GH149986 {
+template <typename T> concept PerfectSquare = [](){} // expected-note 2{{here}}
+([](auto) { return true; }) < PerfectSquare <class T>;
+// expected-error@-1 {{declaration of 'T' shadows template parameter}} \
+// expected-error@-1 {{a concept definition cannot refer to itself}}
+}