[clang] [clang] [Serialization] Introduce noload_redecls (PR #170823)

[Alexander Kornienko via cfe-commits]

alexfh wrote:

The infinite loop seems to be happening in this code:
```
* thread #1, name = 'clang-debug', stop reason = step over
    frame #0: 0x0000555567ade9c8 clang-debug`clang::ASTDeclMerger::MergeDefinitionData(this=0x00007ffffffe0320, D=0x000030c271c7e908, MergeDD=0x000030c271c801f0) at ASTReaderDecl.cpp:2115:19
   2112     unsigned OldLoadedSize = Reader.getNumDeclsLoaded();
   2113 #endif
   2114
-> 2115     for (auto *RD : Def->noload_redecls())
   2116       cast<CXXRecordDecl>(RD)->DefinitionData = ⅅ
```

https://github.com/llvm/llvm-project/pull/170823