[InferAddressSpaces] Support assumed addrspaces from addrspace predicates.

- CUDA cannot associate memory space with pointer types. Even though Clang could add extra attributes to specify the address space explicitly on a pointer type, it breaks the portability between Clang and NVCC.
- This change proposes to assume the address space from a pointer from the assumption built upon target-specific address space predicates, such as `__isGlobal` from CUDA. E.g.,

```
  foo(float *p) {
    __builtin_assume(__isGlobal(p));
    // From there, we could assume p is a global pointer instead of a
    // generic one.
  }
```

This makes the code portable without introducing the implementation-specific features.

Note that NVCC starts to support __builtin_assume from version 11.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D112041