Suppress deprecation warnings the closest to the deprecated code, using
`CURL_IGNORE_DEPRECATION()`. Then drop build-specific suppressions, and
file-wide ones. The latter is not compatible with Unity mode. Also
replace manual suppressions with a macro to apply to all compilers with
deprecation warning support. Also enable deprecation warnings for clang.
- curl/curl.h: enable deprecation warnings for clang.
- docs/examples: stop setting `CURL_DISABLE_DEPRECATION` with autotools.
Suppression moved to C-level earlier. Syncs with cmake.
Follow-up to 5fc61a37c1#14123
- tests/http/clients: stop setting `CURL_DISABLE_DEPRECATION` in
autotools. If it becomes necessary in the future, it can be done in
C via the macro. Syncs with cmake.
- lib1545: stop setting `CURL_DISABLE_DEPRECATION` in autotools.
Drop guard from test source.
Follow-up to 0f10360073#12444
- libtest, unit: replace `CURL_DISABLE_DEPRECATION` with
`CURL_IGNORE_DEPRECATION()`.
- docs/examples: replace pragmas with `CURL_IGNORE_DEPRECATION()`.
Closes#14789
Before this patch, the result code was a mixture of `int` and
`CURLcode`.
Also adjust casts and fix a couple of minor issues found along the way.
Cherry-picked from #13489Closes#13600
