No longer accepting attribute spellings with prefix and suffix underscores except for GNU attributes, or C++11-style attributes in the GNU namespace. This prevents attributes such as __declspec(__dllexport__) or [[__noreturn__]] from being treated as known attributes.
[ms-abi][cleanup] Removing FinalizeCXXLayout : no functional change

After r196549 there is no need to separate FinalizeCXXLayout and
FinalizeLayout so they were merged and FinalizeCXXLayout was eliminated.