[clang] [WIP][C++20][Modules] Lazily, but fully load 'HeaderFileInfo' table into memory. (PR #140867)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

>  It turns out to be kind of a pain to use with HeaderFileInfoTrait. More importantly, we can't really afford to key by the internal key type since that only hashes on the size of the file and the hash collision gets pretty bad. Moreover, the merging and condensing strategy seems rather simple.

Maybe we can use hash value of the input as the key?

https://github.com/llvm/llvm-project/pull/140867