[clang] [clang][modulemap] Lazily load module maps by header name (PR #181916)

[Jan Svoboda via cfe-commits]

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@@ -2261,6 +2261,10 @@ GlobalModuleIndex *CompilerInstance::loadGlobalModuleIndex(
   // we need to make the global index cover all modules, so we do that here.
   if (!HaveFullGlobalModuleIndex && GlobalIndex && !buildingModule()) {
     ModuleMap &MMap = getPreprocessor().getHeaderSearchInfo().getModuleMap();
+
+    // Load parsed but unloaded modules from the module maps.
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jansvoboda11 wrote:

Not sure we use "unloaded" anywhere else. How about this?

```suggestion
    // Load modules that were parsed from module maps but not loaded yet.
```

Also, why is this even necessary? The code below only handles modules deserialized from a PCM file, which this call should not affect.

https://github.com/llvm/llvm-project/pull/181916