gcc: guard #pragma diagnostic in core code for <4.6, disable picky warnings

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 #20366
Fixes #20892
Fixes #20924
Closes #20902
Closes #20907

lib/content_encoding.c

@@ -31,13 +31,13 @@
 #endif
 
 #ifdef HAVE_BROTLI
-#if defined(__GNUC__) || defined(__clang__)
+#ifdef CURL_HAVE_DIAG
 /* Ignore -Wvla warnings in brotli headers */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wvla"
 #endif
 #include <brotli/decode.h>
-#if defined(__GNUC__) || defined(__clang__)
+#ifdef CURL_HAVE_DIAG
 #pragma GCC diagnostic pop
 #endif
 #endif