Match cl.exe's mangling for decomposition declarations.
Decomposition declarations are considered to be anonymous structs,
and use the same convention as for anonymous struct/union declarations.
Naming confirmed to match https://godbolt.org/z/K2osJa
Patch from Eric Astor <epastor@google.com>!
Differential Revision: https://reviews.llvm.org/D67202
llvm-svn: 371124
Nico Weber