clang has had full and tested support for C++0x trailing-return-type and auto type deduction since r126166. Update the website to reflect this, and add a __has_feature test.

trailing-return-type codegen is not tested yet (name mangling in particular).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@128787 91177308-0d34-0410-b5e6-96231b3b80d8