[clang] [clang:frontend] Move helper functions to common location for SemaSPIRV (PR #125045)

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Sirraide wrote:

> `Sema` or `SemaBase`?

Typically we just put those in `Sema`; I think the only thing in `SemaBase` at the moment are some diagnostics helpers.

https://github.com/llvm/llvm-project/pull/125045