[ODRHash] Detect mismatches in anonymous `RecordDecl`.

Allow completing a redeclaration check for anonymous structs/unions
inside `RecordDecl`, so we deserialize and compare anonymous entities
from different modules.

Completing the redeclaration chain for `RecordDecl` in
`ASTContext::getASTRecordLayout` mimics the behavior in
`CXXRecordDecl::dataPtr`. Instead of completing the redeclaration chain
every time we request a definition, do that right before we need a
complete definition in `ASTContext::getASTRecordLayout`.

Such code is required only for anonymous `RecordDecl` because we
deserialize named decls when we look them up by name. But it doesn't
work for anonymous decls as they don't have a name. That's why need to
force deserialization of anonymous decls in a different way.

rdar://81864186

Differential Revision: https://reviews.llvm.org/D140055