[clang] warn_cstruct_memaccess and warn_cxxstruct_memaccess are too agressive about initializers
These warnings are triggered for zeroing initializers of non-trivially
initializable and non-trivially copyable types.
This results in significant numbers of warnings in idiomatic object
initialization code, where memset and similar are used to ensure no
stale data is present in fields or padding bytes.
Addresses #156996