Extend `#pragma diagnostic push`/`pop` guards to the whole codebase
(from tests and examples only) to disable it for GCC <4.6. Rename guard
to `CURL_HAVE_DIAG` and make it include llvm/clang to be interchangeable
with `__GNUC__ || __clang__` in this context.
The above means no longer disabling certain warnings locally, so pair
this with disabling all picky warnings for GCC <4.6.
Also:
- drop global workarounds for misbehaving GCC <4.6 compiler warnings.
Not needed with picky warnings disabled.
Reported-by: fds242 on github
Reported-by: Sergey Fedorov
Thanks-to: Orgad Shaneh
Follow-up to f07a98ae11#20366Fixes#20892Fixes#20924Closes#20902Closes#20907