Fix negative number literal typeannotations

[danez]

Q	A
Bug fix?	y
Breaking change?	n
New feature?	n
Deprecations?	n
Spec compliancy?	y
Tests added/pass?	y
Fixed tickets
License	MIT

Currently negative number literal type annotations contain the wrong raw value:

http://astexplorer.net/#/gist/f6cb8e0c848b800f6fdf2e1b64cad131/90e051a59cdf7e4fa88e79c22f6567dc1c451e45

The raw value is "1" instead of "-1". The flow parser is doing the correct thing.

Also use parseLiteral() to parser string and number literal typeannotations
so that future changes (estree) to literals are also reflected to flow.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@57aacea). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #366   +/-   ##
=========================================
  Coverage          ?   97.85%           
=========================================
  Files             ?       21           
  Lines             ?     3495           
  Branches          ?      927           
=========================================
  Hits              ?     3420           
  Misses            ?       29           
  Partials          ?       46

Impacted Files	Coverage Δ
src/parser/node.js	100% <100%> (ø)
src/plugins/flow.js	98.08% <100%> (ø)
src/parser/expression.js	97.34% <100%> (ø)

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