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

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.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D151281