Support forced unwrapped type as function return type

[Matejkob]

Add support to forced unwrapped types as function return type such as this one:

@spyable protocol Foo {
  func foo() -> String!
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.49%. Comparing base (e8e5595) to head (f4aee27).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
+ Coverage   97.92%   98.49%   +0.57%     
==========================================
  Files          18       18              
  Lines         772      799      +27     
==========================================
+ Hits          756      787      +31     
+ Misses         16       12       -4     

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