Bad parsing behaviour for generic arrow type arguments

[JLHwung]

// No parse error.
f< <T>(v: T) => T>();

// Parse error. 
f<<T>(v: T) => T>();

Flow version: 0.169.0

Expected behavior

No parse errors. They are equivalent.

Actual behavior

Parsing errors.

• Link to Try-Flow or Github repo: https://flow.org/try/#0PTAEDkHtQBwQwE4GcCmoUIZBA6AUAGYA8oRAKgHwAUAbgFyhkCUoAvBY9UwNx54igAColTpM2HKEJFy1eoxbtOVHkA
• Exactly same TS issue: Bad parsing behavior for generic arrow type arguments microsoft/TypeScript#23996

Found this issue when working on a similar fix to Babel (babel/babel#14145).