Introduce a function conversion for "adjusted" dynamic member references.
When we have adjusted the type of a dynamic member reference (e.g.,
for `@objc optional` methods) due to `@preconcurrency` or similar,
form the original dynamic member reference based on the original type
and then introduce an appropriate conversion. This better reflects the
adjustment in the AST, eliminating a SIL verifier crash.
Fixes rdar://94597323.