[Clang][NFC] Refactor `Targets.h` to make it publicly accessible (#116090)

This PR is motivated by the requirements of ClangIR, which includes
compilation pipelines that do not always start from the Clang driver. In
these cases, accessing some target-specific information, such as
obtaining a data layout string for a given target triple or querying
other target details, requires foundational infrastructure like
`clang::TargetInfo`. Since ClangIR is actively being upstreamed, sharing
this logic across components has become essential, which leads to this
PR.

The function `clang::targets::AllocateTarget` serves as the factory for
Clang's `TargetInfo`. To enable sharing, this PR moves `AllocateTarget`
to a public header.

The existing header `clang/lib/Basic/Targets.h` previously contained two
parts: the `AllocateTarget` function and target-specific macro helpers.
With `AllocateTarget` moved, only the macro stuff remain in `Targets.h`.
To better organize the code, the macro helpers have been relocated to a
new file, `clang/lib/Basic/TargetDefines.h` (essentially a rename). The
original `Targets.h` now serves as a proxy header that includes both
headers to maintain compatibility.

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