Introduce two new OpenACC operation interfaces for identifying global
variables and their address computations:
- `GlobalVariableOpInterface`: Identifies operations that define global
variables. Provides an `isConstant()` method to query whether the global
is constant.
- `AddressOfGlobalOpInterface`: Identifies operations that compute the
address of a global variable. Provides a `getSymbol()` method to
retrieve the symbol reference.
This is being done in preparation for `ACCImplicitDeclare` pass which
will automatically ensure that `acc declare` is applied to globals when
needed.
The following operations now implement these interfaces:
- `memref::GlobalOp` implements `GlobalVariableOpInterface`
- `memref::GetGlobalOp` implements `AddressOfGlobalOpInterface`
- `fir::GlobalOp` implements `GlobalVariableOpInterface`
- `fir::AddrOfOp` implements `AddressOfGlobalOpInterface`
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