[clang] [clang][ASTImporter] fix assert fail due to offset overflow (PR #79084)

[Balázs Kéri via cfe-commits]

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@@ -9896,9 +9903,13 @@ Expected<FileID> ASTImporter::Import(FileID FromID, bool IsBuiltin) {
         // FIXME: The filename may be a virtual name that does probably not
         // point to a valid file and we get no Entry here. In this case try with
         // the memory buffer below.
-        if (Entry)
+        if (Entry) {
+          if (isBufferSizeOverflow(ToSM, Entry->getSize()))
+            return llvm::make_error<ASTImportError>(
+                ASTImportError::UnsupportedConstruct);
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balazske wrote:

This error is clearly not an unsupported AST construct. For this case a new error type should be added like `ASTImportError::SourceLocationOverflow` or `ResourceOverflow`. Code in `CrossTranslationUnitContext::importDefinitionImpl` should be updated to count this new error.
Alternatively the `llvm_unreachable` from `UnknownError` can be removed. But `UnknownError` should be used if something happens that is not supposed to happen, even if now it is used like "other error".

https://github.com/llvm/llvm-project/pull/79084