C++ Interop: Support getting void* from C++ functions and passing void* it to C++ function (#6279)

This defines `Cpp.void` as a custom type.
`Cpp.void*` is mapped to C++ `void*`.

Not supported yet: Conversions from and to other pointer types.

C++ Interop Demo:

```carbon
// main.carbon

library "Main";

import Core library "io";

import Cpp inline '''
#include <cstdio>

auto GetPointer() -> void* _Nonnull {
  static int x = 8;
  return &x;
}

auto GetValue(void* _Nonnull ptr) -> int {
  return *static_cast<int*>(ptr);
}
''';

fn Run() -> i32 {
  let ptr: Cpp.void* = Cpp.GetPointer();
  Core.Print(Cpp.GetValue(ptr));
  return 0;
}
```

```shell
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link main.o --output=demo
$ ./demo
8
```

Part of #6280.

toolchain/check/cpp/import.cpp

@@ -1137,6 +1137,9 @@ static auto MapBuiltinType(Context& context, SemIR::LocId loc_id,
               context.ints().Add(ast_context.getTypeSize(qual_type))));
     }
     // TODO: Handle floating-point types that map to named aliases.
+  } else if (type.isVoidType()) {
+    return ExprAsType(context, Parse::NodeId::None,
+                      SemIR::CppVoidType::TypeInstId);
   }
 
   return TypeExpr::None;
@@ -2116,6 +2119,7 @@ static auto LookupBuiltinTypes(Context& context, SemIR::LocId loc_id,
           .Case("float", ast_context.FloatTy)
           .Case("double", ast_context.DoubleTy)
           .Case("long_double", ast_context.LongDoubleTy)
+          .Case("void", ast_context.VoidTy)
           .Default(clang::QualType());
   if (builtin_type.isNull()) {
     return SemIR::InstId::None;