[C++20] [Modules] [Serialization] Deserialize

LValuePathSerializationHelper's type properly

Close https://github.com/llvm/llvm-project/issues/58716.

Tested with libcxx's modules build.

When we read the type of
LValuePathSerializationHelper, we didn't read the correct type. We read
the element type as its name suggests. But the problem here is that it
looks like that both the usage and serialization use its type as the
top level type. So here is the mismatch.

Actually, the type of LValuePathSerializationHelper is never used after
Deserialization without the assertion. So it doesn't matter for the
release users. And this patch shouldn't change the behavior too.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D139406