[clang] [clang][modules] Deserialize submodules lazily (PR #194968)

[Jan Svoboda via cfe-commits]

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@@ -222,6 +230,41 @@ struct ModuleAttributes {
         NoUndeclaredIncludes(false) {}
 };
 
+/// A reference to either a fully materialized Module object, or
+/// a yet-to-be-deserialized submodule in an AST file.
+class ModuleRef {
+  mutable Module *Existing = nullptr;
+  mutable ExternalSubmoduleSource *ExternalSource = nullptr;
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jansvoboda11 wrote:

You're right, I think the follow-up commit changes `operator*` to not clear these, but that would increase traffic through the virtual function for later calls to `operator*`. I pushed a change that uses one bit in `ExternalSource` to signify whether it's been used to deserialize yet. This prevents calling the virtual function repeatedly, but also keeps `ExternalSource` around for use in the follow-up commit.

https://github.com/llvm/llvm-project/pull/194968