[libc] remove BlockStore from cpp namespace (#85312)

The cpp namespace should only be used to mirror APIs from C++'s std::
namespace
(at least until we share more code with libc++, see

https://discourse.llvm.org/t/rfc-project-hand-in-hand-llvm-libc-libc-code-sharing/77701)

GitOrigin-RevId: 6503eff6d3bcaf1158dc7e2b6d1fc3521833374e
3 files changed