Added 'builtinPlacements' prop

[akharin]

Added customization for 'builtinPlacements' prop of internal rc-trigger component.

[ant-design-bot]

Deploy preview for rc-select failed.

Built with commit 0c1f9d2b08e14bc9a94d462e87481632b4f9962c

https://app.netlify.com/sites/rc-select/deploys/5c018a4e41174f38f2d6c378

[codecov-io]

Codecov Report

Merging #354 into master will increase coverage by 0.02%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #354      +/-   ##
==========================================
+ Coverage   97.06%   97.09%   +0.02%     
==========================================
  Files           8        9       +1     
  Lines         307      310       +3     
  Branches       97       97              
==========================================
+ Hits          298      301       +3     
  Misses          8        8              
  Partials        1        1

Impacted Files	Coverage Δ
src/Select.jsx	98.48% <ø> (ø)	:arrow_up:
src/placements.js	100% <100%> (ø)
src/PropTypes.js	100% <100%> (ø)	:arrow_up:
src/SelectTrigger.jsx	97.61% <100%> (ø)	:arrow_up:

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