Support and test opaque pointers

[maleadt]

This PR adds initial support for opaque pointers, and testing of that through JULIA_LLVM_ARGS.

I'm also removing the Manifest as I'm fed up with trying to support older Manifests on newer versions of Julia.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (83b5b74) 83.31% compared to head (25d5ecd) 84.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #528      +/-   ##
==========================================
+ Coverage   83.31%   84.45%   +1.13%     
==========================================
  Files          24       24              
  Lines        3291     3313      +22     
==========================================
+ Hits         2742     2798      +56     
+ Misses        549      515      -34     

Files	Coverage Δ
src/metal.jl	72.86% <100.00%> (+0.17%)	⬆️
src/rtlib.jl	95.18% <100.00%> (+0.18%)	⬆️
src/spirv.jl	93.79% <90.00%> (+12.10%)	⬆️
src/irgen.jl	94.26% <91.66%> (+1.05%)	⬆️
src/driver.jl	89.32% <66.66%> (+0.65%)	⬆️

... and 5 files with indirect coverage changes

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