[LIBC][NFC] Rename errno and assert files to match other files with functions

Rename the files containing the `__errno_location` function
to `__errno_location.h/cpp` to match the other files and move
the `llvmlibc_errno` macro to its own file.

Split assert.h into `__assert_fail.h` (contains the function prototype)
and assert.h (contains the assert macro).

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D90653