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Nicholas Bishop
afd679129d
In generate_ast.cpp, an `CarbonExternalASTSource` is installed that has a `Check::Context` pointer. During lowering, this `ExternalASTSource` is still installed, and using it can cause a crash if the now-invalid pointer is dereferenced. Fix by adding a new `ReadOnlyASTSource` in sem_ir, and using that during lowering. `CarbonExternalASTSource` now inherits from `ReadOnlyASTSource` to avoid some code duplication. In generate_ast.cpp, we now always install a multiplex source, even if there's only one child source. Clang internally keeps pointers to the top-level `ExternalASTSource` installed via `setExternalSource`, and those pointers aren't updated if `setExternalSource` is called again. By using `MultiplexExternalSemaSource`, we can keep the top-level `ExternalASTSource` pointer the same, and only update its children. Using `MultiplexExternalSemaSource` this way requires a new constructor and a method to modify its child sources; added a new LLVM patch adding those. https://github.com/carbon-language/carbon-lang/issues/7142