[clang] Remove #undef alloca workaround (#81534)

Added in 26670dcb to workaround #4885.

Windows CI and a local Windows build are happy with this change, so it
seems like this has been properly fixed at some point. If this does
break somebody, this can be easily reverted. (Also, Linux does the same
`#define alloca` in system headers, so I'm not sure why it'd be
different on Windows)

This is tech debt that caused breakages, see comments on #71709.