Don't crash on an invalid trailing return type on a function before a '...'

clang tries to produce a helpful diagnostic for the traiilng '...', but the
code that r216778 added for this doesn't expect an invalid trailing return type.
Add code to explicitly handle this.

Having explicit code for this but not for other things looks a bit strange, but
trailing return types are special in that they have a separate existence bit in
addition to the type (see r158348).


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@224974 91177308-0d34-0410-b5e6-96231b3b80d8
Driver: unify addClangRT{Linux,Windows}

The differences are pretty superficial:
- .lib vs .a extensions
- whether or not to link (potentially) incorrectly against libgcc_s

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@224975 91177308-0d34-0410-b5e6-96231b3b80d8