types: share Function payload on copy

[rieske]

Summary

• Function copies now share an immutable impl (shared_ptr) instead of deep-cloning the return type and every argument.
• Pointer, array, and record payloads already shared. Function was the remaining expensive Type copy.
• Assignment still replaces the impl, so later writes to one copy do not change the other. Function has no setters.

Isolated measure on this machine (Release, git -O0, 568 TUs): analyze 49s -> 40s, full user 224s -> 209s. diff.c analyze 96ms -> 80ms.

Test plan

• ctest -R '^(typesTest|astTest|symbolsTest|semantic_analyzerTest)$'
• ctest -L att -j24
• CI

[rieske]

Review (cannot request-changes on our own PR)

The share belongs on Function. Pointer, array, and record payloads already share; this was the remaining expensive Type copy. No spaghetti, no file-size issue, no branching. The test locks the assignment-independence that sharing has to preserve.

The pimpl already bought something this patch does not use.

unique_ptr<Type> members existed because Function.h cannot see a complete Type (Type.h includes Function.h). Impl lives only in the .cpp, which already includes Type.h. Keeping unique_ptr inside Impl is the header-cycle workaround transplanted into the one place it is no longer needed.

Store values in Impl:

struct Function::Impl {
    Type returnType;
    std::vector<Type> arguments;
    bool variadic { false };
};

Then:

• construction can be one make_shared<Impl>(...) instead of default-Impl plus three assignments
• getReturnType() / getArguments() can return const Type& / const std::vector<Type>& instead of allocating a vector and cloning every argument on every inspect (sameShape and makeComposite both do this today)
• a function type is one heap object (the impl), not impl + return + each argument

The public ctor can keep taking unique_ptr<Type> so the header cycle stays solved. Deref into Impl at the boundary.

That is the same change, with the incidental heap shape deleted. I would not merge the unique_ptr-in-Impl version.

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[coveralls]

coverage: 94.028% (-0.002%) from 94.03% — types/share-function-payload into master