Match solc diagnostic levels

[seanyoung]

The following errors are warnings in solc

• unreachable code should be warning
• function types with parameter/return names should be warning
• unknown assembly flags should be warnings

[codecov[bot]]

Codecov Report

Merging #1586 (0328fb2) into main (fe66202) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1586   +/-   ##
=======================================
  Coverage   87.57%   87.57%           
=======================================
  Files         133      133           
  Lines       64212    64217    +5     
=======================================
+ Hits        56231    56236    +5     
  Misses       7981     7981           

Files	Coverage Δ
src/codegen/cfg.rs	93.96% <100.00%> (+0.01%)	:arrow_up:
src/codegen/statements/mod.rs	96.78% <100.00%> (+<0.01%)	:arrow_up:
src/sema/namespace.rs	89.61% <100.00%> (-0.02%)	:arrow_down:
src/sema/statements.rs	93.80% <100.00%> (+<0.01%)	:arrow_up: