Implement function-parameter-list and friends in lib/Syntax

Implements the following grammar productions:

- function-parameter-list
- function-parameter

This is mostly reusable for other flavors of function declarations,
such as initializers and whatnot, but those will have separate
top-level syntax nodes.

https://bugs.swift.org/browse/SR-4067

include/swift/Syntax/ExprSyntax.h

@@ -51,6 +51,7 @@ public:
 };
 
 class ExprSyntax : public Syntax {
+  friend class FunctionParameterSyntax;
 public:
   using DataType = ExprSyntaxData;