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

[Alexander Kornienko via cfe-commits]

alexfh wrote:

> I've done some manual build graph cleanup and got the makefile in the test case from a few thousand down to a handful of rules. The reduction started running a bit faster since then: the size of inputs is now ~65MB spread across ~4.5K files. Looking at how it goes, I may have a shareable test case by the end of the week.

10MB and 2.7K files

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