
This PR makes it so that `CompilerInvocation` needs to be provided to `CompilerInstance` on construction. There are a couple of benefits in my view: * Making it impossible to mis-use some `CompilerInstance` APIs. For example there are cases, where `createDiagnostics()` was called before `setInvocation()`, causing the `DiagnosticEngine` to use the default-constructed `DiagnosticOptions` instead of the intended ones. * This shrinks `CompilerInstance`'s state space. * This makes it possible to access **the** invocation in `CompilerInstance`'s constructor (to be used in a follow-up).
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