[clang][OpenMP] Keep 'requires' directives read from an AST file (#220058)

An OpenMP `requires` directive is recorded in Sema when the directive is
parsed
(`SemaOpenMP::ActOnOpenMPRequiresDirective`). Nothing repopulated that
list from an AST
file. A translation unit that gets its `requires` directive from a PCH
or a module
therefore behaves as if the directive were absent, and clang rejects
valid code.

`OMPRequiresDecl` is already serialized and eagerly deserialized, so the
declaration is
present in the AST. Only Sema's view of it was missing.

## Reproducer

```c++
// rev.h
#pragma omp requires reverse_offload
void foo();
```

```c++
// use.cpp
void bar(int argc) {
#pragma omp target device(ancestor : argc)
  foo();
}
```

```
$ clang -cc1 -fopenmp -fopenmp-version=51 -fopenmp-targets=x86_64 -triple x86_64 \
        -x c++ -emit-pch -o r.pch rev.h
$ clang -cc1 -fopenmp -fopenmp-version=51 -fopenmp-targets=x86_64 -triple x86_64 \
        -include-pch r.pch -fsyntax-only use.cpp
use.cpp:2:20: error: device clause with ancestor device-modifier used without specifying 'requires reverse_offload'
```

The requirement is specified, so the diagnostic is a false positive. The
same code in a
single file compiles cleanly, because the directive is then parsed in
this TU.

`-fopenmp-targets=` matters. Without an offload target clang defers this
diagnostic and
then drops it, because a host compilation never flushes its deferred
diagnostics. That is
a separate bug, with a fix to follow, and it makes this false positive
reachable in the
default configuration too.

`reverse_offload` is the case with a directly observable diagnostic. The
same state feeds
every `hasRequiresDeclWithClause` query, so `dynamic_allocators` (on a
device compilation)
and `unified_shared_memory` (which changes implicit data-sharing instead
of emitting a
diagnostic) read it as well.

## Fix

Write the `requires` directives into the AST file and register them with
Sema, following
the existing `DeclsWithEffectsToVerify` pattern. This happens in
`ASTReader::UpdateSema`
rather than `ASTReader::InitializeSema` because `UpdateSema` runs after
every AST file is
loaded. A directive can come from a module loaded during the compilation
rather than up
front.

Tests cover both the PCH and the module path.

The contents of this PR is created with help from Claude Code and
ChatGPT.

---------

Co-authored-by: Alexey Bataev <a.bataev@gmx.com>