[clang][ssaf][NFC] Avoid incomplete EntitySummary type breakage (#182946)
When parsing LUSummary.h as a standalone header unit, EntitySummary is
an incomplete type, causing compilation to fail:
```
__memory/unique_ptr.h:72:19: error: invalid application of 'sizeof' to an incomplete type 'clang::ssaf::EntitySummary'
72 | static_assert(sizeof(_Tp) >= 0, "cannot delete an incomplete type");
...
clang/include/clang/Analysis/Scalable/EntityLinker/LUSummary.h:48:12: note: in instantiation of member function 'std::map<clang::ssaf::SummaryName, std::map<clang::ssaf::EntityId, std::unique_ptr<clang::ssaf::EntitySummary>>>::map' requested here
48 | explicit LUSummary(NestedBuildNamespace LUNamespace)
| ^
clang/include/clang/Analysis/Scalable/EntityLinker/LUSummary.h:27:7: note: forward declaration of 'clang::ssaf::EntitySummary'
27 | class EntitySummary;
```
This is not a total breakage because this header file builds
successfully when used in a .cpp file that includes EntitySummary.h
prior to this.
See https://llvm.org/docs/CodingStandards.html#self-contained-headers
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