cpp-build-env: Install LLVM dev package

[chfast]

I need this to use the same docker image for EVMJIT builds.

[codecov-io]

Codecov Report

Merging #4699 into develop will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #4699      +/-   ##
===========================================
- Coverage    60.86%   60.85%   -0.01%     
===========================================
  Files          343      343              
  Lines        27085    27086       +1     
  Branches      2784     2785       +1     
===========================================
- Hits         16484    16482       -2     
- Misses        9614     9615       +1     
- Partials       987      989       +2

Impacted Files	Coverage Δ
libdevcore/RLP.cpp	75.55% <0%> (-1.78%)	⬇️
libp2p/NodeTable.cpp	52.16% <0%> (-0.82%)	⬇️
libethereum/State.cpp	71.94% <0%> (+0.07%)	⬆️
libethereum/BlockChain.cpp	64.69% <0%> (+0.37%)	⬆️
libp2p/RLPXFrameCoder.cpp	82.55% <0%> (+0.67%)	⬆️

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