Add intrinsic support for atomic operations.

[lucteo]

Add intrinsic support for atomic store and load, atomic Read-Modify-Write, CompareExchange and fence operations.

[codecov]

Codecov Report

Attention: Patch coverage is 85.76419% with 163 lines in your changes missing coverage. Please review.

Project coverage is 88.34%. Comparing base (734e7be) to head (a3fd992).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/CodeGen/LLVM/Transpilation.swift	60.29%	162 Missing ⚠️
Sources/FrontEnd/BuiltinFunction.swift	99.61%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1628      +/-   ##
==========================================
- Coverage   88.44%   88.34%   -0.11%     
==========================================
  Files         381      381              
  Lines       30359    31503    +1144     
==========================================
+ Hits        26851    27830     +979     
- Misses       3508     3673     +165     

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[kyouko-taiga]

I strongly suspect that we can find a simpler way to represent the types of non-returning instructions. What if we say that the return type of a built-in instruction is an enum:

enum BuiltinReturnType {
  case builtin(BuiltinType)
  case void
}

I'll try to propose another implementation ASAP.

[kyouko-taiga]

Looks good mostly. Would be nice if we had more tests covering code generation.

[lucteo]

Thank you very much, will add more tests in a follow-up PR.