misc: upgrade to LLVM 12.0.1

[Wodann]

Updates LLVM to 12.0.1. As LLVM 11.0.1 has been pulled from apt in favour of 11.1.0, we need to upgrade.

This will also unblock the release of Mun 0.3.1 - containing bug fixes.

[codecov]

Codecov Report

Merging #351 (2cfe06a) into main (76d2ee1) will decrease coverage by 0.01%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main     #351      +/-   ##
==========================================
- Coverage   82.55%   82.54%   -0.02%     
==========================================
  Files         264      264              
  Lines       15291    15294       +3     
==========================================
  Hits        12624    12624              
- Misses       2667     2670       +3     

Impacted Files	Coverage Δ
crates/mun_codegen/src/code_gen/symbols.rs	79.14% <0.00%> (+0.48%)	⬆️
crates/mun_codegen/src/ir.rs	41.66% <ø> (ø)
crates/mun_codegen/src/ir/dispatch_table.rs	80.95% <ø> (ø)
crates/mun_codegen/src/ir/ty.rs	75.19% <0.00%> (-1.19%)	⬇️
crates/mun_codegen/src/value/function_value.rs	83.33% <50.00%> (ø)
crates/mun_codegen/src/ir/body.rs	81.53% <66.66%> (-0.25%)	⬇️
crates/mun_codegen/src/intrinsics/macros.rs	100.00% <100.00%> (ø)

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[baszalmstra]

Moved this to 0.4.0 because the core team can't develop without it!

[Wodann]

@baszalmstra all issues have been fixed. Ready for review :)