Fix stack-use-after-scope in CheckExplicitlyDefaultedMemberExceptionSpec

[vitalybuka]

Summary

Similar to r288685.
getExceptionSpec returned structure with pointers to temporarily object created
by computeImplicitExceptionSpec.

[rsmith]

Phabricator doesn't let me retroactively approve, but LGTM, thanks!

lib/Sema/SemaDeclCXX.cpp

void Sema::CheckExplicitlyDefaultedMemberExceptionSpec(
   if (SpecifiedType->getExceptionSpecType() == EST_Unparsed)
     SpecifiedType =
         MD->getTypeSourceInfo()->getType()->castAs<FunctionProtoType>();
 
   // Compute the implicit exception specification.
   CallingConv CC = Context.getDefaultCallingConvention(/*IsVariadic=*/false,
                                                        /*IsCXXMethod=*/true);
   FunctionProtoType::ExtProtoInfo EPI(CC);
-  EPI.ExceptionSpec = computeImplicitExceptionSpec(*this, MD->getLocation(), MD)
-                          .getExceptionSpec();
+  auto IES = computeImplicitExceptionSpec(*this, MD->getLocation(), MD);
+  EPI.ExceptionSpec = IES.getExceptionSpec();
   const FunctionProtoType *ImplicitType = cast<FunctionProtoType>(
     Context.getFunctionType(Context.VoidTy, None, EPI));
 
   // Ensure that it matches.
   CheckEquivalentExceptionSpec(
     PDiag(diag::err_incorrect_defaulted_exception_spec)
       << getSpecialMember(MD), PDiag(),
     ImplicitType, SourceLocation(),