Fix regression in r172376. Don't try to detect missing 'constexpr' specifiers
on redeclarations, since that makes us pick wrong prior declarations under
some circumstances.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172384 91177308-0d34-0410-b5e6-96231b3b80d8
Add extra tests for [[gnu::...]] attributes, missed from r172382.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172391 91177308-0d34-0410-b5e6-96231b3b80d8