Limit Clang diagnostic suppressions to fmt declarations

[yqtian-se]

Clang diagnostic pragmas remain in effect until another pragma changes or
restores them. Two public fmt headers suppress warnings for fmt declarations
but do not restore the diagnostic state before returning to consumer code:

• format.h ignores -Wweak-vtables before declaring format_error;
• std.h ignores -Wbit-int-extension before declaring the bitint and
  ubitint aliases.

Because neither suppression has a matching push and pop, declarations after
the include inherit it. For example, this consumer class normally triggers
-Wweak-vtables, but the warning disappears after including format.h:

#include <fmt/format.h>

class ConsumerClass {
 public:
  virtual void method() {}
};

The same occurs for a consumer _BitInt alias after including std.h:

#include <fmt/std.h>

template <int N>
using consumer_bitint = _BitInt(N);

The requested -Wbit-int-extension diagnostic is suppressed along with fmt's
own aliases. This branch is enabled by default with Clang 15 and later; I used
FMT_USE_BITINT=1 to exercise the same branch with the available Clang 14
compiler.

The matched probes produced the following results:

Consumer declaration	Without the fmt header	Current main	This change
Weak-vtable class with -Wweak-vtables	warning	no diagnostic	warning
_BitInt alias with -Wbit-int-extension	warning	no diagnostic	warning

Thus, including either header currently disables a warning that the consumer
explicitly requested for its own code. In a build that promotes the warning to
an error, the header also changes whether that consumer code is accepted.

This change brackets each suppression with fmt's existing Clang diagnostic
push/pop macros. The warnings remain suppressed for the fmt declarations that
need them, while the consumer's previous diagnostic state is restored
immediately afterward. The macros remain no-ops for non-Clang compilers.

Validation:

• a Clang 14 build with FMT_PEDANTIC=ON and FMT_WERROR=ON succeeds;
• the configured test suite passes 26 of 26 tests.

[vitaut]

Merged, thanks

[yqtian-se]

Thanks!