Add support for RefCountPtr<const T>.
Add support for RefCountPtr<const T>.

`Unref` is const, so we might as well accept `const` pointers in the `RefCountDeleter`. This aligns `RefCountPtr` with `ScopedUnref`, and enables `RefCountPtr` to be used with `const` pointers.

PiperOrigin-RevId: 426474399
Change-Id: I2aa8879c99b0b26d039668fb173ed32e810c6d9c

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