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

docs/examples/sendrecv.c

@@ -34,7 +34,7 @@
  * warning: conversion to 'long unsigned int' from 'curl_socket_t' {aka 'int'}
  * may change the sign of the result [-Wsign-conversion]
  */
-#ifdef __GNUC__
+#ifdef __GNUC__  /* keep outside functions and without push/pop for GCC <4.6 */
 #pragma GCC diagnostic ignored "-Wsign-conversion"
 #elif defined(_MSC_VER)
 #pragma warning(disable:4127)  /* conditional expression is constant */