Instead, check them and their error handling right away.
In addition to fixing the crash in the radar, this also causes
us to emit unused variable warnings in functions containing
local functions.
Eventually, TC.definedFunctions should go away altogether.
Fixes <rdar://problem/53956342>.
Allow the use of declarations whose names start with $ in all
modes. However, normal code cannot define new entities with names that
start with $: only the implementation can do that, e.g., for property
delegates.
When in Swift 3 Compatibility Mode we now acceptable a standalone
'$' as an identifier. In all other cases this is now disallowed
and must be surrounded by backticks.
