[clang] [clang][DependencyScanning] Track dependencies from prebuilt modules to determine IsInStableDir (PR #132237)

[Cyndy Ishida via cfe-commits]

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@@ -5834,9 +5834,13 @@ bool ASTReader::readASTFileControlBlock(
           break;
         case INPUT_FILE:
           bool Overridden = static_cast<bool>(Record[3]);
+          size_t FilenameLen = ModuleDir.size() + Record[7] + 1;
           auto Filename = ResolveImportedPath(PathBuf, Blob, ModuleDir);
+          StringRef FilenameAsRequested = Filename->substr(0, FilenameLen);
+          StringRef ExternalFilename = Filename->substr(FilenameLen);
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cyndyishida wrote:

> IIRC Blob is the concatenation of the as-requested name and the name. 

That is correct. 

Basically `Record[7]` encodes the length of the basename (or filename without the full path) of the vfs version of the file.  https://github.com/llvm/llvm-project/blob/main/clang/lib/Serialization/ASTWriter.cpp#L1902

`ModuleDir` is the directory of the vfs-version of the directory those vfs mapped files live in.  https://github.com/llvm/llvm-project/pull/132237/files#diff-c61a3cce4bfa099b5af032fa83cbf1563f0af4bf58dc112b39571d74b6b681c1R5784 

With that combination I derived the full length of of the vfs file path. The external path is the rest.

https://github.com/llvm/llvm-project/pull/132237