This patch initializes the `ptr` dialect directories and base files,
adding the `!ptr.ptr` type and the `#ptr.spec<...>` data layout spec
attribute.
The `!ptr.ptr` type is an opaque pointer type optionally parameterized
by a memory space. This type typically represents a handle to an object
in memory or target-dependent values like `nullptr`.
The implementation of the `DataLayoutTypeInterface` interface for
`!ptr.ptr` was adapted from `!llvm.ptr`'s implementation. This
implementation uses the `#ptr.spec<...>` attribute for defining the data
layout specification.
See [[RFC] `ptr` dialect & modularizing ptr ops in the LLVM
dialect](https://discourse.llvm.org/t/rfc-ptr-dialect-modularizing-ptr-ops-in-the-llvm-dialect/75142)
for rationale and roadmap.
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