[CodeGen] Assign locations to calls to special struct helpers

Assign artificial locations to calls to special struct-related helper
functions.

Such calls may not inherit a location if emitted within FinishFunction,
at which point the lexical scope stack may be empty, causing CGDebugInfo
to report the current DebugLoc as empty.

Fixes an IR verifier complaint about a call to '__destructor_8_s0' not
having a !dbg location attached.

rdar://57293361