[reference-binding] Fix an ambiguity when parsing inout in function types.

This was exposed by:

decl/enum/enumtest.swift

The problem here is that we are now allowing for inout to be the start of a
swift decl. This means that if one defines an enum case with a inout parameter,
the parser gets confused and doesnt think it is part of the type. I worked
around this by changing canParseTupleTypeBody to say that an inout binding
cannot be parsed as part of a tuple type. This fits already with how we want to
model this.

Author: gottesmm

Sources/SwiftParser/Types.swift

@@ -783,7 +783,10 @@ extension Parser.Lookahead {
 
   mutating func canParseTupleBodyType() -> Bool {
     guard
-      !self.at(.rightParen, .rightBrace) && !self.atContextualPunctuator("...") && !self.atStartOfDeclaration()
+      !self.at(.rightParen, .rightBrace) && !self.atContextualPunctuator("...") &&
+      // In types, we do not allow for an inout binding to be declared in a
+      // tuple type.
+      (self.at(.keyword(.inout)) || !self.atStartOfDeclaration())
     else {
       return self.consume(if: .rightParen) != nil
     }