[clang] [lldb] [Clang][SPIRV] Add __spirv_event_t builtin type (PR #207077)

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elizabethandrews wrote:

> We should validate that it can round trip serialization correctly, 

Can you clarify what you mean by this. I test serialization in `AST/spirv-event-type.cpp`, similar to what is done here -  https://github.com/llvm/llvm-project/pull/94830

> test mangling behavior (oops, probably need to update the demangler as well), etc.

Mangling has been tested in `CodeGenCXX/spirv-event-type.cpp`. I am not sure how to write a test for demangling. I could not find tests for opaque builtins I based this patch on. FWIW I tested locally and it works. We don't need to update the demangler since we use u<len><name> mangling and this is already handled by the demangler. 

```
$ ./build/bin/llvm-cxxfilt _Z12consumeEventu15__spirv_event_t
consumeEvent(__spirv_event_t)

$ ./build/bin/llvm-cxxfilt _Z4funcIu15__spirv_event_tEvT_
void func<__spirv_event_t>(__spirv_event_t)
```




https://github.com/llvm/llvm-project/pull/207077