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

[Jan Svoboda via cfe-commits]

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@@ -48,9 +48,17 @@ namespace clang {
 
 class FileManager;
 class LangOptions;
+class Module;
 class ModuleMap;
 class TargetInfo;
 
+/// Interface for on-demand deserialization of submodules stored in a PCM file.
+class ExternalSubmoduleSource {
+public:
+  virtual Module *getSubmodule(uint32_t GlobalID) = 0;
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jansvoboda11 wrote:

I agree that's marginally better name, but there's already `ASTReader::getSubmodule()` that I wanted to reuse. That avoids the need to either update existing call sites to from `getSubmodule()` to `readSubmodule()` (or have `ASTReader::getSubmodule()` just come a shim that forwards to `ExternalSubmoduleSource::readSubmodule()`).

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