[libc] [llvm] [libc] Move libc_errno.h to libc/src/__support and make LIBC_ERRNO_MODE_SYSTEM to be header-only. (PR #143187)

[via llvm-commits]

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@@ -46,4 +46,7 @@ LIBC_CONFIGURE_OPTIONS = [
 
     # Documentation in libc/src/__support/libc_assert.h
     # "LIBC_COPT_USE_C_ASSERT",
+
+    # Use system errno.
+    "LIBC_ERRNO_MODE=LIBC_ERRNO_MODE_SYSTEM",
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lntue wrote:

The problem was that originally (and even right now), only the entry points have separate objects that are built with and without `-DLIBC_COPT_PUBLIC_PACKAGING`.  And other object libraries (from `add_object_library`) were flip-flopping between having internal targets or not, with or without `-DLIBC_COPT_PUBLIC_PACKAGING` or not.  And the `errno` which was evolved into `libc_errno` to address different needs, ended up with different configurations for `-DLIBC_COPT_PUBLIC_PACKAGING`, and it was and odd-ball, being an entrypoint (even though the standard has `errno` as a macro), and allowed other objects / entrypoints to depend on.

And those were hard to untangle without too much rewriting most of the cmake builds and not affecting downstream non-cmake users.  So we decided to let all the internal usages of `errno` use `LIBC_NAMESPACE::libc_errno` instead, and let the `libc_errno` entrypoint objects deal with different configurations.

My upcoming cmake fix / refactor will make the roles of each cmake targets clearer and more consistent, allowing us to get away from the current indirection of the original `LIBC_ERRNO_MODE_SYSTEM`.

In the mean time, I would call the clean `#define libc_errno errno` mode `LIBC_ERRNO_MODE_SYSTEM_INLINE`, and preserve the `LIBC_ERRNO_MODE_SYSTEM` as before.

https://github.com/llvm/llvm-project/pull/143187