Lazily provide a __float128 dummy type in -std=gnu++11 mode.

This is needed to parse libstdc++ 4.7's type_traits, see PR13530.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184476 91177308-0d34-0410-b5e6-96231b3b80d8