mimalloc-override.h does not work correctly with namespaces

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Hello! Recently, I decided to build a project with Clang using mimalloc. I opted for overriding through header files, as I believe this is the most reliable and controllable way to override calls. The flags are quite simple for this:

```
C*FLAGS="-include mimalloc-override.h"
LDFLAGS="-lmimalloc"
```

However, Clang failed to compile with errors

```
/usr/lib/llvm/21/include/llvm/Support/MemAlloc.h:53:18: error: no member named 'mi_realloc' in namespace 'std'; did you mean simply 'mi_realloc'?
53 | void *Result = std::realloc(Ptr, Sz);
| ^ ~~~~~~~~~~~~~~~~
/usr/include/mimalloc.h:111:40: note: 'mi_realloc' declared here
```

It seems that the method of overriding using `#define` does not account for calls that may use namespaces (or their aliases). For now, one option I see is to add definitions of `mi_realloc` into the `std` namespace, creating something like this:

**mimalloc-override.h**
```cpp
#include

#define malloc(n) mi_malloc(n)

#if defined(__cplusplus)
namespace std {
inline auto mi_malloc(size_t n) { return ::mi_malloc(n); }
}
#endif
```

**out.cpp**
```cpp
#include "mimalloc-override.h"
#include

int main() {
auto _ = std::malloc(8);
}
```

**preprocessed_out.cpp**
```cpp
# 2 "./mimalloc-override.h" 2

namespace std {
inline auto mi_malloc(size_t n) { return ::mi_malloc(n); }
}
# 2 "out.cpp" 2

int main() {
auto _ = std::mi_malloc(8);
}
```

Thank you for your time and for the great project! :)

Start with mimalloc-override.h and reproduce the failure using the -include mimalloc-override.h and -lmimalloc flags, then inspect the namespace-qualified calls shown from llvm/Support/MemAlloc.h. Verify the override behavior for std::malloc and std::realloc, and add coverage demonstrating that the affected Clang compilation succeeds.

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