[ThinLTO] Fix empty .llvmcmd sections

When passing -lto-embed-bitcode=post-merge-pre-opt, we were getting
empty .llvmcmd sections. It turns out that is because the
CodeGenOptions::CmdArgs field was only populated when clang saw
-fembed-bitcode={all|marker}.

This patch always populates the CodeGenOptions::CmdArgs. The overhead
of carrying through in memory in all cases is likely negligible in
the grand schema of things, and it keeps the using code simple.

Differential Revision: https://reviews.llvm.org/D90366