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

[Maksim Ivanov via cfe-commits]

emaxx-google wrote:

> Two observations:
> 
> 1. There is a diamond module structure: Both `4BK.pcm` and `LUM.pcm` depend on `WI9.pcm` and contain `2OT.h` with different module names (I removed transitive empty includes). I'm not sure if this is valid... @emaxx-google is it possible to share the modulemap for Abseil? We should check if all private headers that are potentially used by multiple "public" modules are correctly taken care of.

Not sure if I get the question correctly, but `2OT.h` is a non-modular textual header. Yes, it happens to be #included from two places which end up compiled into different modules, but that should be OK?..

These modules don't originate from Abseil: `WI9.pcm` comes from STL; `4BK.pcm` and `LUM.pcm` are some google3 libraries.

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