[libc] casting macro for user headers

[nickdesaulniers]

@enh-google saw our endian.h and mentioned __BIONIC_CAST.
https://android.googlesource.com/platform/bionic/+/main/libc/include/sys/cdefs.h#57

#if defined(__cplusplus)
#define __BIONIC_CAST(_k,_t,_v) (_k<_t>(_v))
#else
#define __BIONIC_CAST(_k,_t,_v) ((_t) (_v))
#endif

maybe interesting to add that to libc/include/__llvm-libc-common.h and use more in our user facing headers.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

@enh-google saw our endian.h and mentioned `__BIONIC_CAST`. https://android.googlesource.com/platform/bionic/+/main/libc/include/sys/cdefs.h#57 ```c++ #if defined(__cplusplus) #define __BIONIC_CAST(_k,_t,_v) (_k<_t>(_v)) #else #define __BIONIC_CAST(_k,_t,_v) ((_t) (_v)) #endif ``` maybe interesting to add that to libc/include/__llvm-libc-common.h and use more in our user facing headers.