[libc] Group guarded functions in generated headers by their guarding macros.

[lntue]

#81010 introduced guarded functions in generated headers, and the macro checks are generated for each function. Ideally, we should group the functions based on their guarding macros, so that in the generated headers, each guarding macro is only checked once.

[llvmbot]

@llvm/issue-subscribers-libc

Author: None (lntue)

https://github.com//pull/81010 introduced guarded functions in generated headers, and the macro checks are generated for each function. Ideally, we should group the functions based on their guarding macros, so that in the generated headers, each guarding macro is only checked once.