[clang] [clang][CodeGen] Omit pre-opt link when post-opt is link requested (PR #85672)

[Jacob Lambert via cfe-commits]

lamb-j wrote:

Switched the option from a cl::opt to a Clang/CodeGen option.

Also added a LIT test which tests both -mlink-builtin-bitcode-postopt and -mno-link-builtin-bitcode-postopt

https://github.com/llvm/llvm-project/pull/85672