[QoI] improve diagnostics for operator declarations; unify parsing code

[jtbandes]

Improves diagnostics for invalid operator declarations, particularly when specifying precedence for a non-infix operator. (Prompted by @erica's post about operators!)

Along the way, cleans up code that was nearly identical between parseDeclOperator{Infix,Prefix,Postfix}(), replacing them with a single function named parseDeclOperatorImpl().

Before:

error: consecutive statements on a line must be separated by ';'
postfix operator +? : DefaultPrecedence
                   ^
                   ;
error: expected expression
postfix operator +? : DefaultPrecedence
                    ^

After:

error: only infix operators may declare a precedence
postfix operator +? : DefaultPrecedence
                    ^~~~~~~~~~~~~~~~~~~

[jtbandes]

@swift-ci please smoke test

[rintaro]

In this PR or later, I suggest to diagnose multiple fixture here.
Currently, postfix infix operator ++++ is accepted.

Although prefix postfix combinations are diagnosed in parseNewDeclAttribute,
we should not simply add DAK_Infix there, because that will emit superfluous diagnostic for func decl:

error: attribute 'infix' cannot be combined with this attribute
infix prefix func ++(x: Int, y: Int) {}
      ^
error: 'infix' modifier is not required or allowed on func declarations
infix prefix func ++(x: Int, y: Int) {}
^~~~~~

[jtbandes]

I think you could probably handle DAK_Infix there, if you made it so that infix is forgotten, rather than kept, after emitting the 'infix' modifier is not required or allowed on func declarations error.

[DougGregor]

If 'infix' modifier is not required or allowed on func declarations' is emitted early enough, marking it invalid will prevent later checks from seeing it. It's fine for that to be a follow-up PR.

[DougGregor]

This LGTM; let's address @rintaro 's comments as a follow-up.