Implement Microsoft-compatible mangling for decomposition declarations.

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