test(code_gen): add incremental compilation test

[Wodann]

For now this is limited to a split between group_ir and file_ir as a group currently exists of a single file. In the future we will need to test that changing one file doesn't cause regeneration of another file.

NOTE: Once we add support for multiple files, we need to do a second optimization round. file_ir depends on group_ir for usage of the DispatchTable, but this could potentially be optimised by adding an external LLVM function that MUST be inlined. If we succeed in this optimization, file_ir never has to be regenerated when another file's changes cause the group_ir to be updated.

[codecov]

Codecov Report

Merging #102 into master will increase coverage by 0.09%.
The diff coverage is 92.59%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   77.78%   77.87%   +0.09%     
==========================================
  Files         134      134              
  Lines        8806     8833      +27     
==========================================
+ Hits         6850     6879      +29     
+ Misses       1956     1954       -2     

Impacted Files	Coverage Δ
crates/mun_codegen/src/test.rs	97.50% <87.50%> (-2.50%)	⬇️
crates/mun_codegen/src/mock.rs	100.00% <100.00%> (ø)
crates/mun_hir/src/ty/primitives.rs	69.56% <0.00%> (+4.34%)	⬆️
crates/mun_hir/src/arena.rs	77.14% <0.00%> (+5.71%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c72b965...52c31f1. Read the comment docs.