Split redefine_extname out of -Wpragmas

Right now, many warnings are controlled by -Wpragmas itself, but #pragma redefine_extname's warning can over-occur in code bases easily.

#pragma redefine_extname foo bar

// ...

namespace x {
    void foo() {}
}

prints:

<source>:6:10: warning: #pragma redefine_extname is applicable to external C declarations only; not applied to function 'foo' [-Wpragmas]
    6 |     void foo() {}
      |          ^

See https://godbolt.org/z/hGsdhaWWM.

When this is intentional, the only way to turn off the warning is to turn the warning down (from a -Werror starting point) at the definition site of foo using:

#pragma GCC diagnostic warning "-Wpragmas"

However, this requires touching the definition sites, which don't even have pragmas, so this is kinda weird.

But as I personally do not even see a point in this warning (we intend to only affect external C symbols after all), I would like to turn it off (or down from error to warning) without affecting all the rest in -Wpragmas. So best would be getting a specific flag for just this one.

A slightly different option would be if the warning could be "back-dated" to the #pragma redefine_extname line (and also be affected by the pushed/popped diagnostics state from there). Then we could disable it in a targeted manner without affecting everything else.

Any ideas?

Start by tracing how Clang handles #pragma redefine_extname diagnostics and how that warning is grouped under -Wpragmas. Compare the separate-warning and back-dated-diagnostic options described in the issue; done means one targeted behavior is implemented and covered by relevant compiler diagnostic tests.