[Concepts] Add missing CXXThisScope to function template constraint substitution

We did not have a CXXThisScope around constraint checking of functions and
function template specializations, causing a crash when checking a constraint
that had a 'this' (bug 44689).

Recommit after fixing test.

clang/lib/Sema/SemaConcept.cpp

@@ -329,6 +329,13 @@ bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
     Satisfaction.IsSatisfied = true;
     return false;
   }
+  Qualifiers ThisQuals;
+  CXXRecordDecl *Record = nullptr;
+  if (auto *Method = dyn_cast<CXXMethodDecl>(FD)) {
+    ThisQuals = Method->getMethodQualifiers();
+    Record = const_cast<CXXRecordDecl *>(Method->getParent());
+  }
+  CXXThisScopeRAII ThisScope(*this, Record, ThisQuals, Record != nullptr);
   // We substitute with empty arguments in order to rebuild the atomic
   // constraint in a constant-evaluated context.
   // FIXME: Should this be a dedicated TreeTransform?

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -4271,7 +4271,13 @@ bool Sema::CheckInstantiatedFunctionTemplateConstraints(
             Scope, MLTAL))
       return true;
   }
-
+  Qualifiers ThisQuals;
+  CXXRecordDecl *Record = nullptr;
+  if (auto *Method = dyn_cast<CXXMethodDecl>(Decl)) {
+    ThisQuals = Method->getMethodQualifiers();
+    Record = Method->getParent();
+  }
+  CXXThisScopeRAII ThisScope(*this, Record, ThisQuals, Record != nullptr);
   return CheckConstraintSatisfaction(Template, TemplateAC, TemplateArgs,
                                      PointOfInstantiation, Satisfaction);
 }

clang/test/SemaTemplate/instantiate-requires-clause.cpp

@@ -57,4 +57,13 @@ struct S3 {
 	static constexpr void f(Args...) { }
 };
 
-static_assert((S3<int>::f(), true));
+static_assert((S3<int>::f(), true));
+
+template<typename T>
+struct S4 {
+    template<typename>
+    constexpr void foo() requires (*this, true) { }
+    constexpr void goo() requires (*this, true) { }
+};
+
+static_assert((S4<int>{}.foo<int>(), S4<int>{}.goo(), true));