Add GCC's noexcept-type alias for c++1z-compat-mangling

Summary: GCC has named this `-Wnoexcept-type`, so let's add an alias to stay compatible with the GCC flags.

Reviewers: rsmith, dexonsmith

Reviewed By: dexonsmith

Subscribers: cfe-commits, karies, v.g.vassilev, ahatanak

Differential Revision: https://reviews.llvm.org/D34439

llvm-svn: 308340