Pitch: The CStdlib module

[stevapple (YR Chen)]

@stevapple

Re-organize system libraries to a Darwin-like module

We have been specializing Glibc and making this name an umbrella for any libc from except Darwin and Windows. However, this is not true. Different libcs provides different sets of functionalities, and we should allow consuming all possible APIs from the system library.

Also, the way we load Glibc today is not standardized, while the Windows practice has already provided a standardized way to do such thing. On how to organize the libc module (maybe better expressed as system SDK module), I suggest referring to Darwin.modulemap since that’s really neat.

I want to especially point out that this step is vital yet very breaking. It will eventually change the system SDK module name for nearly every target that’s not using Glibc. To be frank, this is the major concern that is driving me to push and land the work before Swift 6.0.