Stop using __attribute__((retain)) in GCC builds (#133793)
GCC sometimes produces warnings about `__attribute__((retain))` despite
`__has_attribute(retain)` being 1. See:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587
The amount of users who benefit from the attribute is probably very
small compared to the amount of `-Werror` enabled builds or the desire
to keep `-Wattributes` enabled in the LLVM build. So for now drop usage
of the `retain` attribute in GCC builds.
