Replace VarDecl::getFirstDecl with VarDecl::getCanonicalDecl

I was having trouble building a C-Reduce with a slightly older version of Clang due to csmith-project/creduce@2417d62c73b17392a5874f32d9ae5653fb0c0fed. It seems that at some point Clang renamed `VarDecl::getFirstDeclaration()` to `VarDecl::getFirstDecl()`.

Now that I understand the root cause of my compilation problem, it's not unreasonable to reject this patch. But `VarDecl::getCanonicalDecl()` works in both versions.