[4.0][index] Add missing parameter and return type references in subscript declarations

[nathawes]

• Explanation: The indexer wasn't covering subscript params or return types, resulting in a rename of any type references there failing and renames initiated on other occurrences of those types not updating them.
• Scope of Issue: Affects rename of symbols used in subscript param/return types.
• Radar: rdar://problem/32314185
• Origination: The indexer was explicitly avoiding indexing subscript indices, though its unclear why from (the 2013 commit message doesn't mention a reason).
• Risk: Low; This patch removes special case handling of subscripts.
• Testing: Added regression tests for these cases

[nathawes]

@swift-ci Please test