[clang] [Serialization] Fix lazy template loading (PR #133057)

[Alexander Kornienko via cfe-commits]

alexfh wrote:

> > So far I found one assertion failure:
> > ```
> > assert.h assertion failed at llvm-project/clang/lib/AST/RecordLayoutBuilder.cpp:3377 in const ASTRecordLayout &clang::ASTContext::getASTRecordLayout(const RecordDecl *) const: D && "Cannot get layout of forward declarations!"
> > ...
> > Trying to produce a test case.
> > ```
> 
> The reduction is running, but it's going to take a lot of time, I'm afraid (as it usually is with module-related bugs). The initial size of the inputs is around 450MB.

The reduction has been crawling for 5 days now. The input is down to 275MB, but the progress has slowed down a lot.

https://github.com/llvm/llvm-project/pull/133057