[SourceKit/Indentation] Fix over-indent after function decl with no param list or body.

[nathawes]

Indentation after an incompletion function declaration with no parameter list or body always indents the next line relative to it, incorrectly.

E.g:

struct Foo {
  bar
    } // this gets indented relative to 'bar'

struct Foo {
  bar
    func foo() {} // this gets indented relative to 'bar'
} 

This is particularly bad because we treat a placeholder in a struct/class body as if it were an incompletion function declaration, and when completing on a struct/class keyword in Xcode you get the snippet below with such a placeholder and the incorrect indentation:

struct <#name#> {
  <#fields#>
    }

Resolves rdar://problem/64618791

[nathawes]

@swift-ci please test