some floating point expressions in unions not done at runtime as required by C

The following code does not conform to C23. This is on both Intel x64 and ARM.

```c
/* clang does automatic union wrong of NaN expression */

/*
C23 N3219
F.8.5 Constant expressions
1 An arithmetic constant expression of floating type, other than one
in an initializer for an object that has static or thread storage
duration or that is declared with storage-class specifier constexpr,
is evaluated (as if) during execution; thus, it is affected by any
operative floating-point control modes and raises floating-point
exceptions as required by ISO/IEC 60559 (provided the state for the
FENV_ACCESS pragma is "on").434)
*/

#include
#include
#include
#pragma STDC FENV_ACCESS ON

#define INV (0.f / 0.f)

static int flags;

static void test1(void){
static union { float f; int i; } uf0 = { INV };
assert( isnan( uf0.f ) );
flags = fetestexcept( FE_ALL_EXCEPT );
assert( 0 == flags ); /* make sure right flag set */
}

static void test2(void){
static const union { float f; int i; } uf0 = { INV };
assert( isnan( uf0.f ) );
flags = fetestexcept( FE_ALL_EXCEPT );
assert( 0 == flags ); /* make sure right flag set */
}

static void test3(void){
const union { float f; int i; } uf0 = { INV };
assert( isnan( uf0.f ) );
flags = fetestexcept( FE_ALL_EXCEPT );
assert( 0 != flags ); /* make sure right flag set */
assert( FE_INVALID == flags ); /* make sure right flag set */
}

static void test4(void){
union { float f; int i; } uf0 = { INV };
assert( isnan( uf0.f ) );
flags = fetestexcept( FE_ALL_EXCEPT );
assert( 0 != flags ); /* make sure right flag set */
assert( FE_INVALID == flags ); /* make sure right flag set */
}

int main(void){
feclearexcept( FE_ALL_EXCEPT );
test1();
feclearexcept( FE_ALL_EXCEPT );
test2();
feclearexcept( FE_ALL_EXCEPT );
test3();
feclearexcept( FE_ALL_EXCEPT );
test4();
return 0;
}
```