[clang] [clang-tools-extra] [lldb] [clang][frontend] Require invocation to construct `CompilerInstance` (PR #137668)

[Michael Spencer via cfe-commits]

https://github.com/Bigcheese approved this pull request.

Makes sense. Every non-test creation of `CompilerInstance` has an associated invocation.

https://github.com/llvm/llvm-project/pull/137668