[clang] [clang] Fix ASTWriter crash after merging named enums (PR #114240)

[Michael Jabbour via cfe-commits]

michael-jabbour-sonarsource wrote:

@vsapsai Thanks again for the feedback :pray: 

> I've realized that the test doesn't have to be Objective-C, the failure is reproducible in C. Curiously, we aren't hitting the assertion in C++/Objective-C++ but I don't know why.

I dug a bit into this today, and I could find a few differences:

1. ODR checks seem to be implemented on C and ObjC only (see df0ee34bc2523f11b907311670c776cb3de108c1). As far as I understand, in the C++ case, `Sema::ActOnTag` sets [`SkipBody.ShouldSkip` to true](https://github.com/llvm/llvm-project/blob/cd708029e0b2869e80abe31ddb175f7c35361f90/clang/lib/Sema/SemaDecl.cpp#L17616), causing `ParseEnumSpecifier` to [skip the body and return early](https://github.com/llvm/llvm-project/blob/cd708029e0b2869e80abe31ddb175f7c35361f90/clang/lib/Parse/ParseDecl.cpp#L5553-L5561) before it would otherwise [call `ParseEnumBody`](https://github.com/llvm/llvm-project/blob/cd708029e0b2869e80abe31ddb175f7c35361f90/clang/lib/Parse/ParseDecl.cpp#L5603) and create the `EnumDeclConstant` AST node (that we need to clean up in the C/Objective-C case).

2. Another difference, which doesn't seem to contribute directly to the divergence in this specific case, is that in C++, `ASTReader` pre-loads a set of interesting identifiers, see 33e0f7ef94124a53404555879666cef0e370f8c2.

> I wanted to try defining the enums in different scopes (e.g. in a struct) and see how it works. Consequences for scopes aren't clear, so that's worth taking another look.

I added one more test case for this. I got the same stack trace, and the crash seems to be fixed after the PR as well. However, I couldn’t reproduce the stack trace with `WriteCUDAPragmas`, so I might be missing something. See the scoped example [on CE](https://godbolt.org/#z:OYLghAFBqd5QCxAFwE4FN0BoCWIIDGAtgIYDW6AgqsAM4gDkAtACIDCAspQNICiA%2BgCEAqgEkAMi34AVAJoAFXgFIAzCxboARgFdgDLAQD2RAA44ANulTiSAO2DaSwdKIAmIAkoBMg74IPatMjGAPLayCbhAGIW6LYkROgo6EFYAGax9LDorjjBqG4gXumxhSollvGJILQIJBiuAHQIWKAM%2Bji0orYE5tquOYwADB20HCQ4tgDKhtqoBEkMAIyjISZxw/rmdsCF7QCUWBA5eYYF7uUZloUALBVxCUnoAB4JJpaNBCYmrYyj3b1%2BoMGCNcGMJtNZvNFqDOmsNiCtjs9gxDsdcvlCkt7oUAGz3KpJIiGVzaD7E1ykH5tf49PoDdyIsHjSYzOYLRgrMHw2ybLDbeyFTw%2BPxok6Y9x3K4udwAdgJjxAHBJlBiliWzV%2B7TBAPpwNh4NZUI5y1W615TIFu3cwt8PjFGLOhQArDj3AAOBXVZWuVWxLyamk6ulAxkGlmQ9mLLlw818q1Cvyio7ip3ufHSwoATi9SR9gjV6EDf2DgIZfM6EbZ0M5ZoRoITNqT9sOnU45HQ8lQhgAVugCMha1geyBsbZ0AB3QtYoaHcyMZ36C2gwyMaQYdAAam8Ki5g8RaKQRgGvEdqFw%2BGlhJqxqLCG%2B6UMtmQUwIJEso6lT%2BQwloneefIjsUVoeM2/gmHY6AAHKKrU9Q5Jqf6WAOOBPvgcSuGwhh9EQvIqHcGHiJM0HaEQmhWEUWAmIYtB5KhthYTheF3NRtHIPRRHjlBpHkaglFIf27FPlMyD1MgjGkcxWACShwmiagyCcSRZEUcUQRiRJuEgPh0nyYpxHcSpfFeIctC3owQSoNoA6bhwACeIlWTZSiyr4QyUJum5xKRtl2bwtikc626ub5ABq778CwwVRTE6DmK4qhuZQLksIlqL8guS58qucENJq%2B4jIeHgkugp6nOeeAQFeiqkJMnwPmk36vu%2BSRLF%2Bz6/v%2BgHafyOygSKPhUZBMHVC8bwfF8PwyUJvLHLYmHYZJo5YIRBk8RRLE0XRT6aby2KsdtthKYZvEgHc030SJGmLVp2IXXJYnHetfHnXpu3LepClPUZZ2meZDA7pMZZbt4Xj5oWzSg0o7nQ5QkzIJutW2BA%2BzBYIm4YMgcy2GFEWpSo6MpbD6Xzgwi5YMu%2BirmNpgTQ%2BBXpRAR4lWV%2BQXlVsTXqaj7Ps1H5tTzP5/vIAFMiO2IgbafhDVxNUkmSRYUlS0lxYJ9HofN73YqtXHPaO%2BIHTNWtUVtM3fadSz4vd0xvTde0q8hM1XV9a0/ZbunXUxH16ebFGW39UaMBSCu2SqaOw55weWKHvqFks4fuZ5nkIOgJADKg25eGDKpx5DWcR0nLzUQpm4AFQF0TieI/L0c%2Bn6lheAnHlJynadWJn2ex/6edeAXnlF2cCPl1XlfJbKqUw%2B5UdbvmTfJ6n6cd%2BDsQ9wXA8l8PY8T5QJOZRT2UgNPjRKyQPwM0Vx6lWe7PVdU3ONbzb78%2B1QtdWLZ29fY/V2uBw2KnXucWjWw1gtL22t5q%2B2MibNi9FjaGw4q7U6alVayRtp7Jad0UFOx9og1SHsFLG0%2BvpXWP0TLSX%2BoDEMAwO65QQggKG7ld5kyykyVcADYgahaOfI4zMTzX0qrfaM%2BgH4vifq1F%2BnURbdVdJLMCMtoL/xzt3IBWD1ZzVARglaEDcFQPgTtO2y09FHR0fxVRD0CEGMwY7S6OCSFIPweJSxDjIFFADjWBgAB6Dxm5eCmGQHZTc0piZzj3pTLAbClEN3ypsC%2BLN%2BGXk5oqe%2BTUxGfkFpI0WoIRz4lkQNX%2BstvQkgLCvFR1i0LqONjrZSp1XRGLgabBBdiKKumts7RxYCHZq3McQ6pzSHGENsb0vizo3Emi8THYp0cBjmlcLQTcT4Y713QAGBA8zbDmDso0WGlDgZL0ics1e7lxm6n6JMYAm5aFNFWSnDAm43yBBSJuKY6BSCbmCIjKwzg3kp03JQKY0hNy2BKnMtI3YiAXLqHlVZE48irOQD8hAOBXADBxk%2BR5oLjCLK2TDLwKggZ6hoZCuhDCd4hOYfvVhSoikQy4TEw4rd074EOGQEAzohgZQYDcfQRBRxDHZWE1c2p6DYm4XAWAMBEDFVMLECq%2BAjDSssHxXoOx%2BAkFoH%2BBS9FaD8DQNoWwZAKjICsPQCAmg%2BSaEmPUOyQ4LXxFQHZEImg%2BwDiHPKxIz4QjrOtUyAYOhgBsHfOYeg3MBikHsB%2BH1OAMAoQAG4pD5C8fs4QhG4GfHFeMOBNCoCtVhAYfI0A4B5dzONqBNA0XQBoMNwBzBnJiekbYdBQo4EnDyIcIi%2BbiPScLTJ%2Bhxaf1AFLQaNbNBZBRuEkwM1g1MGeHczcTA0jTzmfO6eTAqTztiKoFgR8T4mDnWkWgdlnwkGeEwJ8GyqYltQEi%2BNkBTJmNmhhd6NxXRVJOhRcodSDHPugYdFx5RWm2y9t%2BgDj0TH/sA0tYDgy318RUIcNAmBRxkvJgKxgm5njulxEwXENw7lWk3BANVGrJ1vKsvq1GEB1yYEzrufY%2Bhz70oXhRMdpMuVYB5Sod0jRcWylxLKJYspMPOndEMG4KhXSoaFSAdloqJVMxALMCI4R2byrMIqkaiwMNYZw3hnYm4lhZkaBqUE7bUkCxERk7qIqNxIuWhObNJhazIZYSuRgYQlMI0MGkdDmHsO4eVfYAjRGrAkd1eRgjWEFXtx3EsOjWAGMcrYzy3EGpeO4q8O6LwSx8IGedPiST%2Bh6AybpVgFlbKOUqBc1TEsCXSZeCq%2BEmrJWS20TQjcIAA%3D).

https://github.com/llvm/llvm-project/pull/114240