[Modules] Fix the inconsistency of which `Decl` should be serialized for an identifier. (#135887)

Fixes the assertion failure
> Assertion failed: (DeclIDs.contains(D) && "Declaration not emitted!"),
function getDeclID, file ASTWriter.cpp, line 6873.

We prepare to serialize a `Decl` by adding it to `DeclIDs` in
`ASTWriter::GetDeclRef`. But the checks before this call aren't the same
as when we are actually serializing a `Decl` in
`ASTIdentifierTableTrait::EmitData` and
`ASTWriter::WriteIdentifierTable`. That's how we can end up serializing
a `Decl` not present in `DeclIDs` and hitting the assertion. With the
assertions disabled clang crashes when trying to use a deserialized null
`Decl`.

Fix by making the code checks before `ASTWriter::GetDeclRef` call
similar to those we have before the serialization.

rdar://139319683

0 files changed, 0 insertions, 0 deletions