[NFC][CLANG] Fix issue with dereference null return value found by Coverity

[Manna]

Summary

Reported by Static Analyzer Tool, Coverity:

Inside "SemaDeclCXX.cpp" file, in clang::​Sema::​CheckExplicitlyDefaultedSpecialMember(clang::​CXXMethodDecl *, clang::​Sema::​CXXSpecialMember, clang::​SourceLocation): Return value of function which returns null is dereferenced without checking.

//returned_null: getAs returns nullptr (checked 117 out of 143 times). 
// var_assigned: Assigning: Type = nullptr return value from getAs.
const FunctionProtoType *Type = MD->getType()->getAs<FunctionProtoType>();

//Dereference null return value (NULL_RETURNS)
//dereference: Dereferencing a pointer that might be nullptr Type when calling getReturnType. 
ReturnType = Type->getReturnType();


//Dereference null return value (NULL_RETURNS)
//dereference: Dereferencing a pointer that might be nullptr Type when calling getParamType. 
QualType ArgType = ExpectedParams ? Type->getParamType(0) : QualType();

This patch uses castAs instead of getAs which will assert if the type doesn't match.

[Manna]

Thank you @erichkeane for reviews!

clang/lib/Sema/SemaDeclCXX.cpp

if (DeleteOnTypeMismatch)
       ShouldDeleteForTypeMismatch = true;
     else {
       Diag(MD->getLocation(), diag::err_defaulted_special_member_variadic)
         << CSM << MD->getSourceRange();
       HadError = true;
     }
   }
 
-  const FunctionProtoType *Type = MD->getType()->getAs<FunctionProtoType>();
+  const FunctionProtoType *Type = MD->getType()->castAs<FunctionProtoType>();
 
   bool CanHaveConstParam = false;
   if (CSM == CXXCopyConstructor)
     CanHaveConstParam = RD->implicitCopyConstructorHasConstParam();
   else if (CSM == CXXCopyAssignment)
     CanHaveConstParam = RD->implicitCopyAssignmentHasConstParam();
 
   QualType ReturnType = Context.VoidTy;