Make number of declared structs contextual

[frabert]

Fixes #293

[github-actions]

See the diff generated by this PR for the tests here: https://github.com/lifting-bits/rellic/actions/runs/2902212007

fizzbuzz.bc

typedefs_of_typedefs.bc

switch_loop.bc

func_cond_zero_arg.bc

fcmp.bc

conflicting_names.bc

struct.bc

issue_183_literal_structs.bc

zeroinit.bc

issue_127_uint128_t_lit.bc

struct_swap.bc

init_list.bc

ret0.bc

bool.bc

short.bc

cast.bc

nullptr.bc

nested_struct.bc

array_swap.bc

binops.bc

bitmask.bc

loop.bc

float.bc

reg_test_structure_fields.bc

vectors.bc

funcptr.bc

goto_loop.bc

branch.bc

bitops.bc

trunc.bc

inttoptr.bc

issue_4.bc

issue_123_uint128_t.bc

fizzbuzz_stateful.bc

issue_94_strncmp.bc

global_using_function_decl.bc

byval_struct.bc

nested_while.bc

func_cond_two_arg.bc

conflicting_global.bc

assert.bc

zext.bc

switch.bc

template_parameter_pack.bc

byval_tail_nogep.ll

byval_tail_gep.ll