[clang-tools-extra] [clangd] [Modules] Support Reusable Modules Builder (PR #106683)

[kadir çetinkaya via cfe-commits]

================
@@ -85,19 +85,20 @@ class PrerequisiteModules {
 /// different versions and different source files.
 class ModulesBuilder {
----------------
kadircet wrote:

can we just introduce a new helper class
```cpp
class ModulesBuilder {
...
private:
  class ModuleFileCache;
  std::unique_ptr<ModuleFileCache> MFCache;
};
```

in `ModulesBuilder.cpp`:
```cpp
class ModuleFileCache {
private:
std::mutex CacheMu;
StringMap<std::weak_ptr<ModuleFile>> Cache;
public:
// Returns cahed `ModuleName` if one exists and it's up-to-date per BuiltModuleFiles.
// It's a prereq for BuiltModuleFiles to contain all the prereqs of ModuleName (i.e. this function is not recursive)
std::shared_ptr<ModuleFile> getOrBuildModule(llvm::StringRef ModuleName,
                            const GlobalCompilationDatabase &CDB,
                            const ThreadsafeFS &TFS, ProjectModules &MDB,
                            PathRef ModuleFilesPrefix,
                            StandalonePrerequisiteModules &BuiltModuleFiles);
};
```

Afterwards we can keep most of the logic as-is. we just need to update `buildModuleFile` to take in a `ModuleFileCache&` as a parameter and call `ModuleFileCache::getOrBuildModule` instead of building one itself.

We also need to verify freshness of a module-file we're going to return from cache in `getOrBuildModule`, we can do so efficiently by only verifying the sources that are directly part of `ModuleName` and not the transitive closure (we can assume all the deps were already checked when putting `BuiltModuleFiles` together).

You can store a pointer to `ModuleFileCache` in `ModuleFile` to trigger deletion from map on destruction of `ModuleFile`s.

https://github.com/llvm/llvm-project/pull/106683