Fix lowering of big structs passed by value

[Jezurko]

• Don't load+store in caller
• Fix loads and stores in callee
• Check if we need additional checks
• Add test:

struct big { int i[sizeof (int) >= 4 && sizeof (void *) >= 4 ? 0x4000 : 4]; };
struct big gb;
int foo (struct big b, int x) { return b.i[x]; }
int main (void) { return foo (gb, 0) + foo (gb, 1); }

[github-actions]

Cpp-Linter Report ⚠️

Some files did not pass the configured checks!

clang-format (v19.1.1) reports: 1 file(s) not formatted

• lib/vast/Conversion/ABI/LowerABI.cpp

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