[5.7] [Sema] Look through FunctionConversionExpr when finding @discardableResult callee

[theblixguy]

Description: When calling a @discardableResult @MainActor annotated function, the call to the function is wrapped in an implicit FunctionConversionExpr. The code was not looking through this expression to find the callee function and thus was incorrectly triggering a result of call to function <name> is unused warning. This warning does not trigger on 5.6, so it is a regression in 5.7.

Risk: Low, a tiny change to the logic to look through this implicit expression
Review by: @xedin
Testing: PR Testing
Original PR: #60324

[theblixguy]

@swift-ci please test

[lukeredpath]

This fix has not been included in the Xcode 14 release candidate, which is a bit of a surprise. Feedback filed FB11474967.

[tkremenek]

I think this fix was right at the edge of the qualification window for 5.7.0, but the 5.7 branch will be the basis of potential minor dot updates.