preserve outer context for await blocks

[Rich-Harris]

fixes #1251

[codecov-io]

Codecov Report

Merging #1379 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1379      +/-   ##
==========================================
+ Coverage   91.11%   91.11%   +<.01%     
==========================================
  Files         120      120              
  Lines        4421     4422       +1     
  Branches     1360     1360              
==========================================
+ Hits         4028     4029       +1     
  Misses        156      156              
  Partials      237      237

Impacted Files	Coverage Δ
src/compile/nodes/AwaitBlock.ts	93.58% <100%> (+0.08%)	⬆️

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