When building the TypeRefinementContext subtree for a function declaration,
postpone creation of the subtree if the function body is unparsed. This allows
the compiler to completely avoid parsing function bodies that have been skipped
(e.g. with -experimental-skip-non-inlinable-function-bodies) while still
ensuring that the TRCs for functions are built lazily later if needed. When
lazily generating SIL for a function with -experimental-lazy-typecheck, the
TRCs must be built out while typechecking the function in order to emit correct
diagnostics and SIL for `if #available` queries.
Resolves rdar://117448323