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>.
- moving WARNING definition from DiagnosticsCommon.def to DiagnosticsSema.def
- improving WARNING message to better align with guidelines described in docs/Diagnostics.md as suggested by @xwu.
- adapting expected warning message in tests.