Properly handle async arrow functions with multiline type parameters

[alangpierce]

Fixes #396

This is similar to another case: we detect it as a special case, then move the (
to right after the async, so that all newlines will be in parens so it won't be
detected as the end of a statement by the parser.

[codecov-io]

Codecov Report

Merging #443 into master will increase coverage by 0.27%.
The diff coverage is 90%.

@@            Coverage Diff            @@
##           master    #443      +/-   ##
=========================================
+ Coverage   81.22%   81.5%   +0.27%     
=========================================
  Files          50      50              
  Lines        5157    5412     +255     
  Branches     1204    1208       +4     
=========================================
+ Hits         4189    4411     +222     
- Misses        673     706      +33     
  Partials      295     295

Impacted Files	Coverage Δ
src/transformers/FlowTransformer.ts	100% <ø> (ø)	⬆️
src/transformers/TypeScriptTransformer.ts	94.93% <ø> (ø)	⬆️
src/transformers/RootTransformer.ts	92.22% <90%> (-0.26%)	⬇️
src/util/formatTokens.ts	70.58% <0%> (-10.06%)	⬇️
src/parser/util/identifier.ts	90.9% <0%> (-4.1%)	⬇️
src/parser/plugins/jsx/index.ts	91.33% <0%> (-1.48%)	⬇️
src/util/getClassInfo.ts	85.12% <0%> (-1.21%)	⬇️
src/parser/traverser/statement.ts	81.02% <0%> (-1.19%)	⬇️
src/identifyShadowedGlobals.ts	87.8% <0%> (-1.09%)	⬇️
src/parser/plugins/flow.ts	63.55% <0%> (-0.87%)	⬇️
... and 21 more

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