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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

The only non-NFC change I'm aware of is that the `err_module_rebuild_finalized` diagnostic is now only emitted by `compileModule()` and not by `CompilerInstance::createModuleFromSource()`. I believe that's fine, since that's only used from `#pragma clang module build/endbuild` intended for debugging, but I can copy it around to remove that difference too.

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