[6.0] Add an APIs to get the date at which the unit file that produced SymbolLocation was modified and the latest modification date of a unit that contains a given source file

[ahoppen]

• Explanation: This allows us to check for out-of-date index entries in sourcekit-lsp.
• Scope: Adds new API to indexstore-db that accesses existing information
• Risk: Very low, this does not modify any existing code
• Testing: Tested that this API provides the correct information in sourcekit-lsp (Implement a syntactic test discovery fallback for XCTests written in Swift sourcekit-lsp#1148)
• Issue: n/a
• Reviewer: @bnbarham on Add an APIs to get the date at which the unit file that produced SymbolLocation was modified and the latest modification date of a unit that contains a given source file #185

[ahoppen]

@swift-ci Please test