[clang][NFC] Introduce `SemaBase` (#87634)

This is a follow-up to #84184. Multiple reviewers there pointed out to
me that we should have a common base class for `Sema` and `SemaOpenACC`
to avoid code duplication for common helpers like `getLangOpts()`. On
top of that, `Diag()` function was requested for `SemaOpenACC`. This
patch delivers both.

The intent is to keep `SemaBase` as small as possible, as things there
are globally available across `Sema` and its parts without any
additional effort from usage side. Overused, this can undermine the
whole endeavor of splitting `Sema` apart.

Apart of shuffling code around, this patch introduces a helper private
function `SemaDiagnosticBuilder::getDeviceDeferredDiags()`, the sole
purpose of which is to encapsulate member access into (incomplete)
`Sema` for function templates defined in the header, where `Sema` can't
be complete.