[clang] [C++20] [Modules] Introduce thin BMI (PR #71622)

[David Blaikie via cfe-commits]

dwblaikie wrote:

The words probably don't need to be short.

Interface BMI
Implementation BMI

Seem like fine, accurate terms. I guess we could say "BMInterface" and "BMImplementation" but probably best to jus tgloss over "I" being "interface" and have "interface BMI" and "implementation BMI"

https://github.com/llvm/llvm-project/pull/71622