Fix OOM in FormatDiagnostic (2nd attempt) (#108866)

Resolves: #70930 (and probably latest comments from clangd/clangd#251)
by fixing racing for the shared DiagStorage value which caused messing wit

Fix OOM in FormatDiagnostic (2nd attempt) (#108866)

Resolves: #70930 (and probably latest comments from clangd/clangd#251)
by fixing racing for the shared DiagStorage value which caused messing with args inside the storage and then formatting the following message with getArgSInt(1) == 2:

def err_module_odr_violation_function : Error<
"%q0 has different definitions in different modules; "
"%select{definition in module '%2'|defined here}1 "
"first difference is "

which causes HandleSelectModifier to go beyond the ArgumentLen so the recursive call to FormatDiagnostic was made with DiagStr > DiagEnd that leads to infinite while (DiagStr != DiagEnd).

The Main Idea:
Reuse the existing DiagStorageAllocator logic to make all DiagnosticBuilders having independent states.
Also, encapsulating the rest of state (e.g. ID and Loc) into DiagnosticBuilder.

The last attempt failed -
https://github.com/llvm/llvm-project/pull/108187#issuecomment-2353122096
so was reverted - #108838

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Revert "Fix OOM in FormatDiagnostic" (#108838)

Reverting due to build failures found in #108187

Fix OOM in FormatDiagnostic (#108187)

Resolves: #70930 (and probably latest comments from
https://github.com/clangd/clangd/issues/251)
by fixing racing for the shared `DiagStorage` value which cau

Fix OOM in FormatDiagnostic (#108187)

Resolves: #70930 (and probably latest comments from
https://github.com/clangd/clangd/issues/251)
by fixing racing for the shared `DiagStorage` value which caused messing
with args inside the storage and then formatting the following message
with `getArgSInt(1)` == 2:
```
def err_module_odr_violation_function : Error<
"%q0 has different definitions in different modules; "
"%select{definition in module '%2'|defined here}1 "
"first difference is "
```
which causes `HandleSelectModifier` to go beyond the `ArgumentLen` so
the recursive call to `FormatDiagnostic` was made with `DiagStr` >
`DiagEnd` that leads to infinite `while (DiagStr != DiagEnd)`.

**The Main Idea:**
Reuse the existing `DiagStorageAllocator` logic to make all
`DiagnosticBuilder`s having independent states.
Also, encapsulating the rest of state (e.g. ID and Loc) into
`DiagnosticBuilder`.

**TODO (if it will be requested by reviewer):**
- [x] add a test (I have no idea how to turn a whole bunch of my
proprietary code which leads `clangd` to OOM into a small public
example.. probably I must try using
[this](https://github.com/llvm/llvm-project/issues/70930#issuecomment-2209872975)
instead)
- [x] [`Diag.CurDiagID !=
diag::fatal_too_many_errors`](https://github.com/llvm/llvm-project/pull/108187#pullrequestreview-2296395489)
- [ ] ? get rid of `DiagStorageAllocator` at all and make
`DiagnosticBuilder` having they own `DiagnosticStorage` coz it seems
pretty small so should fit the stack for short-living
`DiagnosticBuilder` instances

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[clang][NFC] Move `PDiag` into `SemaBase` (#93849)

This patch moves `PDiag` into `SemaBase`, making it readily available
everywhere across `Sema` without `SemaRef`, like the regular `Diag`.

[clang] Introduce `SemaCUDA` (#88559)

This patch moves CUDA-related `Sema` function into new `SemaCUDA` class,
following the recent example of SYCL, OpenACC, and HLSL. This is a part
of the effort

[clang] Introduce `SemaCUDA` (#88559)

This patch moves CUDA-related `Sema` function into new `SemaCUDA` class,
following the recent example of SYCL, OpenACC, and HLSL. This is a part
of the effort to split Sema. Additional context can be found in
https://github.com/llvm/llvm-project/pull/82217,
https://github.com/llvm/llvm-project/pull/84184,
https://github.com/llvm/llvm-project/pull/87634.

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[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 c

[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.

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