[Parse] Add back TrailingSemiLoc to Expr, Stmt, and Decl

[rintaro]

Follow up on #6005 (comment) .

Add back TrailingSemiLoc to Expr, Stmt, and Decl.
To correctly accomplish this, I modified parseDecl() to return ParserResult<Decl> with the significant decl. For instance, let (x, y) = (1, 2); produces 3 decls:

(pattern_binding_decl
  (pattern_tuple (pattern_named 'a') (pattern_named 'b'))
  (tuple_expr ...))
(var_decl "a")
(var_decl "b")

Before #6005, the trailing semi-colon was attached to (var_decl "b") the last decl.
In this change, parseDecl() returns PatternBindingDecl so that the semi-colon can be attached to it.

Also, modified ASTDumper to print trailing_semi for Expr, Stmt, and Decl if they have trailing semi-colon.

[rintaro]

CC @bitjammer

[rintaro]

@swift-ci Please smoke test

[bitjammer]

SUPER.

[bitjammer]

Thank you, @rintaro!