[clang] [clang] Extract `CompilerInstance` creation out of `compileModuleImpl()` (PR #134887)

[Michael Spencer via cfe-commits]

Bigcheese wrote:

Can you not just move the `err_module_rebuild_finalized` diagnostic into `compileModuleImpl`? It has access to everything it needs there.

https://github.com/llvm/llvm-project/pull/134887