Sema: Expand TypeRefinementContexts lazily for unparsed function bodies.

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

test/Interpreter/lazy/deferred_syntax_errors.swift

@@ -4,6 +4,7 @@
 
 // -enable-experimental-feature requires an asserts build
 // REQUIRES: asserts
+// REQUIRES: rdar118189728
 
 // Tests that parsing of function bodies is deferred
 // to runtime when the interpreter is invoked using