[6.0] Make Symbol conform to Hashable

[ahoppen]

• Explanation: Don’t repeat a function in incomingCalls if it contains multiple calls to the same function sourcekit-lsp#1166 creates a dictionary with Symbol as a key, which requires Symbol to conform to Hashable
• Scope: Adding Hashable conformance to Symbol
• Risk: Very low, just adding a protocol conformance
• Testing: Verified that Symbol can be used as a dictionary key with this change
• Issue: n/a
• Reviewer: @bnbarham on Make Symbol conform to Hashable #187