[ThinLTO] add post-thinlto-merge option to -lto-embed-bitcode

This will embed bitcode after (Thin)LTO merge, but before optimizations.
In the case the thinlto backend is called from clang, the .llvmcmd
section is also produced. Doing so in the case where the caller is the
linker doesn't yet have a motivation, and would require plumbing through
command line args.

Differential Revision: https://reviews.llvm.org/D87636