libstdc++ 4.4 uses __is_signed as an identifier, while Clang treats it

as a keyword for the __is_signed type trait. Cope with this conflict
via some hackish recovery: if we see a declaration of the form

 static const bool __is_signed

then we stop treating __is_signed as a keyword and instead treat it as
an identifier. It's ugly, but it's better than making the __is_signed
type trait conditional on some language flag. Fixes PR9804.

llvm-svn: 130399