Fix findFunctionType for OrTypes

[odersky]

Restore funtionality that was dropped in 6ac8b47

Fixes #15460

[som-snytt]

 Wrong number of errors encountered when compiling out/compileNeg/neg/i11694
expected: 4, actual: 6
Unfulfilled expectations:
Unexpected errors:
tests/neg/i11694.scala:8
tests/neg/i11694.scala:9
-> following the errors:
 at 8: Missing parameter type
I could not infer the type of the parameter x.
 at 9: Missing parameter type
I could not infer the type of the parameter x.
 at 16: Missing parameter type
I could not infer the type of the parameter x.
 at 17: Missing parameter type
I could not infer the type of the parameter x.
 at 18: Missing parameter type
I could not infer the type of the parameter x.
 at 19: Found:    Int => Int
Required: (Int => Int) & (Int => Int) & Unit