Fix solution application for keypath function subtype conversions

In #39612 we added subtyping for keypaths-as-functions, but during application
the implementation naively coerced the keypath expression itself to the
inferred supertype, resulting in erroneous results. This patch updates the
solution application logic to build the keypath-function conversion expression
based entirely on the 'natural' keypath type, only converting to the inferred
supertype at the end via the usual coerceToType machinery for function
conversions.

test/SILGen/keypaths.swift

@@ -637,3 +637,18 @@ struct TestKeyPathWithSomeType : DefineSomeType {
 
   }
 }
+
+// apple/swift#71423
+protocol CodingKey {}
+
+struct URICoderCodingKey : CodingKey {}
+
+struct CodingStackEntry {
+   var key: URICoderCodingKey
+}
+
+struct Test {
+  var codingStack: [CodingStackEntry]
+  var codingPath: [any CodingKey] { codingStack.map(\.key) }
+  // CHECK: keypath $KeyPath<CodingStackEntry, URICoderCodingKey>, (root $CodingStackEntry; stored_property #CodingStackEntry.key : $URICoderCodingKey)
+}